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    <h1>Auroræ : their characters and spectra</h1>




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    <p>Project Gutenberg's Auroræ: Their Characters and Spectra, by J. Rand Capron</p>


    <p>This eBook is for the use of anyone anywhere in the United States and most
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    <p>Title: Auroræ: Their Characters and Spectra<br/>

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  <!-- page half title _________________________________________________________ -->
  <section class="frontmatter" data-type="half-title">
    <h1>Auroræ<br />
    <span class="smaller">Their characters and spectra</span></h1>
  </section>

  <!-- page half title _________________________________________________________ -->
  <section class="frontmatter" data-type="titlepage">

    <h1>Auroræ<span class="smaller">Their characters and spectra</span></h1>

    <p id="author"><span>By</span> J. Rand Capron, F.R.A.S</p>


    <div id="editor">
    <p class="editor">London<br/>E. &amp; F. N. Spon, 46 Charing cross</p>

    <p class="editor">New York<br/>446 Broom Street</p>

    <p class="editor">1879</p>
  </div>


  </section>

  <!-- page copyright _________________________________________________________ -->
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        <img src="assets/aurorae/images/alere.jpg" width="150" height="150" alt="ALERE FLAMMAM." />

      <p>
        Printed by Taylor and Francis<br/>
        Red Lion Court, Fleet Street</p>
    </div>
  </section>


  <!-- page dedication_________________________________________________________ -->
  <section class="frontmatter" data-type="dedication">
    <p>To Prof. Charles Piazzi Smyth, F.R.S.E.,<br/>
      astronomer royal for Scotland,<br/>
      one of the earliest spectroscopic observers<br/>
      of the aurora and zodiacal light,<br/>
      this volume is respectfully dedicated <br/>by the author.
    </p>
  </section>


  <section class="frontmatter" data-type="epigraph">
    <blockquote>
      <p>
        And now the Northern Lights begin to burn, faintly at first, like sunbeams playing in the waters of the blue sea. Then a soft crimson glow tinges the heavens. There is a blush on the cheek of night. The colours come and go; and change from crimson to gold, from gold to crimson. The snow is stained with rosy light. Twofold from the zenith, east and west, flames a fiery sword; and a broad band passes athwart the heavens, like a summer sunset. Soft purple clouds come sailing over the sky, and through their vapoury folds the winking stars shine white as silver. With such pomp as this is Merry Christmas ushered in, though only a single star heralded the first Christmas.
      </p>

      <p class="caption">Longfellow</p>

    </blockquote>
  </section>








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  </section>





  <!-- page preface_________________________________________________________ -->
  <section class="frontmatter preface">
    <h2>Preface</h2>
    <p class="shorter">Preface</p>

     <p>
       Probably few of the phenomena of Nature so entirely charm and interest scientific and non-scientific observers alike as the Aurora Borealis, or “Northern Lights” as it is popularly called. Whether contemplated as the long low quiescent arc of silver light illuminating the landscape with a tender radiance, as broken clouds and columns of glowing ruddy light, or as sheaves of golden rays, aptly compared by old writers to aerial spears, such a spectacle cannot fail at all times to be a subject of admiration, in some cases even of awe.
     </p>
     <div id="test-bleed"></div>
     <p>
       Hence it is no wonder that the Aurora has always received a considerable amount of attention at the hands of scientific men. Early explorers of the Arctic Regions made constant and important observations of it and its character; and the list of references to works given in the Appendix will show how often it formed the subject of monographs and communications to learned Societies. The early contributions seem relatively more numerous than those of a later date; and the substance of them will be found well summed up in Dr. Brewster’s ‘Edinburgh Encyclopædia’ (1830), article “Aurora.” A most complete and able epitome of our more recent experience and knowledge of the Aurora and its spectrum has been contributed by my friend Mr. Henry R. Procter to the present (9th) edition of the ‘Encyclopædia Britannica,’ article “Aurora Polaris.” It is, however, a drawback to Encyclopædic articles that their matter is of necessity condensed, and that they rarely have the very desirable aid of drawings and engravings to illustrate their subjects. In spite, therefore, of the exhaustive way, both as to fact and theory, in which the contributor to the ‘Encyclopædia Britannica’ has realized his task, it seemed to me there was still room left for a popular treatise, having for its object the description of Auroræ, their characters and spectra. The question of the Aurora spectrum seems the more worthy of extended discussion in that it still remains an unsolved problem. In spite of the observations and researches of Ångström, Lemström, and Vogel abroad, and of Piazzi Smyth, Herschel, Procter, Backhouse, and others at home, the goal is not yet reached; for while the faint and more refrangible lines are but doubtfully referred to air, the bright and sharp red and green lines, which mainly characterize the spectrum, are as yet unassociated with any known analogue.
    </p>
    <p>
      With these views, and to incite to further and closer observations, I have been induced to publish the present volume as a sort of Auroral Guide. For much of the history of the Aurora I am indebted to, and quote from former articles and records, including the two excellent Encyclopædic ones before referred to. Mr. Procter, Mr. Backhouse, and my friend Mr. W. H. Olley have each kindly furnished me with much in the way of information and suggestion. Dr. Schuster has lent me tubes showing the true oxygen spectrum; while Herr Carl Bock, the Norwegian naturalist, has enabled me to reproduce a veritable curiosity, viz. a picture in oil painted by the light of a Lapland Aurora. The experiments detailed in Part III. were suggested by the earlier ones of De la Rive, Varley, and others, and demonstrate the effect of the magnet on electric discharges. For assistance in these I am indebted to my friend Mr. E. Dowlen.
    </p>
    <p>
      The illustrations are mainly from original drawings of my own. Those from other sources are acknowledged. Messrs. Mintern have well reproduced in chromo-lithography the coloured drawings illustrating the Auroræ, moon-patches, &amp;c.
    </p>
  </section>

  <!-- page introduction _________________________________________________________ -->
  <section class="frontmatter introduction">

    <h2>Introduction</h2>
    <p class="shorter">Introduction</p>
     <p>
       Probably few of the phenomena of Nature so entirely charm and interest scientific and non-scientific observers alike as the Aurora Borealis, or “Northern Lights” as it is popularly called. Whether contemplated as the long low quiescent arc of silver light illuminating the landscape with a tender radiance, as broken clouds and columns of glowing ruddy light, or as sheaves of golden rays, aptly compared by old writers to aerial spears, such a spectacle cannot fail at all times to be a subject of admiration, in some cases even of awe.
     </p>
     <p>
       Hence it is no wonder that the Aurora has always received a considerable amount of attention at the hands of scientific men. Early explorers of the Arctic Regions made constant and important observations of it and its character; and the list of references to works given in the Appendix will show how often it formed the subject of monographs and communications to learned Societies. The early contributions seem relatively more numerous than those of a later date; and the substance of them will be found well summed up in Dr. Brewster’s ‘Edinburgh Encyclopædia’ (1830), article “Aurora.” A most complete and able epitome of our more recent experience and knowledge of the Aurora and its spectrum has been contributed by my friend Mr. Henry R. Procter to the present (9th) edition of the ‘Encyclopædia Britannica,’ article “Aurora Polaris.” It is, however, a drawback to Encyclopædic articles that their matter is of necessity condensed, and that they rarely have the very desirable aid of drawings and engravings to illustrate their subjects. In spite, therefore, of the exhaustive way, both as to fact and theory, in which the contributor to the ‘Encyclopædia Britannica’ has realized his task, it seemed to me there was still room left for a popular treatise, having for its object the description of Auroræ, their characters and spectra. The question of the Aurora spectrum seems the more worthy of extended discussion in that it still remains an unsolved problem. In spite of the observations and researches of Ångström, Lemström, and Vogel abroad, and of Piazzi Smyth, Herschel, Procter, Backhouse, and others at home, the goal is not yet reached; for while the faint and more refrangible lines are but doubtfully referred to air, the bright and sharp red and green lines, which mainly characterize the spectrum, are as yet unassociated with any known analogue.
    </p>
    <p>
      With these views, and to incite to further and closer observations, I have been induced to publish the present volume as a sort of Auroral Guide. For much of the history of the Aurora I am indebted to, and quote from former articles and records, including the two excellent Encyclopædic ones before referred to. Mr. Procter, Mr. Backhouse, and my friend Mr. W. H. Olley have each kindly furnished me with much in the way of information and suggestion. Dr. Schuster has lent me tubes showing the true oxygen spectrum; while Herr Carl Bock, the Norwegian naturalist, has enabled me to reproduce a veritable curiosity, viz. a picture in oil painted by the light of a Lapland Aurora. The experiments detailed in Part III. were suggested by the earlier ones of De la Rive, Varley, and others, and demonstrate the effect of the magnet on electric discharges. For assistance in these I am indebted to my friend Mr. E. Dowlen.
    </p>
    <p>
      The illustrations are mainly from original drawings of my own. Those from other sources are acknowledged. Messrs. Mintern have well reproduced in chromo-lithography the coloured drawings illustrating the Auroræ, moon-patches, &amp;c.
    </p>
  </section>

  <!-- List of plates_________________________________________________________
  <section class="frontmatter list-plates">
    <h2>List of plates</h2>

    <table summary="List of plates" id="plates">
      <tr>
        <th class="tdr smaller">Plate.</th>
        <th></th>
        <th></th>
        <th></th>
      </tr>
      <tr>
        <td class="tdr">I.</td>
        <td>The Aurora during the Ice-pressure</td>
        <td class="nw vb"><i>To face page</i></td>
        <td class="tdr vb"><a href="#plate1">14</a></td>
      </tr>
      <tr>
        <td class="tdr">II.</td>
        <td>Aurora seen by Dr. Hayes, 6th January, 1861</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate2">16</a></td>
      </tr>
      <tr>
        <td class="tdr">III.</td>
        <td>Aurora, Guildford, Oct. 24, 1870</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate3">18</a></td>
      </tr>
      <tr>
        <td class="tdr">IV.</td>
        <td>Aurora, Guildford, Feb. 4, 1872; Eclipsed Moon, Aug. 23, 24, 1877</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate4">20</a></td>
      </tr>
      <tr>
        <td class="tdr">V.</td>
        <td>Corona, Graphical Auroræ, Zodiacal Light, &amp;c.</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate5">21</a></td>
      </tr>
      <tr>
        <td class="tdr">VI.</td>
        <td>Aurora, Guildford, Feb. 4, 1874; Spectrum des Nordlichts (Vogel)</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate6">22</a></td>
      </tr>
      <tr>
        <td class="tdr">VII.</td>
        <td>Aurora, Kyle Akin, Isle of Skye, Sept. 11, 1874</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate7">24</a></td>
      </tr>
      <tr>
        <td class="tdr">VIII.</td>
        <td>Herr Carl Bock’s Lapland Aurora, Oct. 3, 1877</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate8">25</a></td>
      </tr>
      <tr>
        <td class="tdr">IX.</td>
        <td>Compared Aurora and other Spectra. Loomis’s curves of Auroras, Magnetic Declination, and Solar Spots</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate9">59</a></td>
      </tr>
      <tr>
        <td class="tdr">X.</td>
        <td>Spectroscope, Micrometer, Tubes</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate10">91</a></td>
      </tr>
      <tr>
        <td class="tdr">XI.</td>
        <td>Aurora-spectra, Candle-spectrum</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate11">102</a></td>
      </tr>
      <tr>
        <td class="tdr">XII.</td>
        <td>Aurora-spectrum, Solar spectrum, and Candle-spectrum</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate12">104</a></td>
      </tr>
      <tr>
        <td class="tdr">XIII.</td>
        <td>Vogel’s Aurora-lines, Aurora-lines near G, and in the red and green</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate13">108</a></td>
      </tr>
      <tr>
        <td class="tdr">XIV.</td>
        <td>Aurora, Hydrocarbons, Oxygen</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate14">110</a></td>
      </tr>
      <tr>
        <td class="tdr">XV.</td>
        <td>Aurora and Air-tubes, &amp;c.</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate15">115</a></td>
      </tr>
      <tr>
        <td class="tdr">XVI.</td>
        <td>Aurora, Phosphoretted Hydrogen, Iron, &amp;c.</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate16">117</a></td>
      </tr>
      <tr>
        <td class="tdr">XVII.</td>
        <td>Effect of Magnet on Tubes and Spark</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate17">134</a></td>
      </tr>
      <tr>
        <td class="tdr">XVIII.</td>
        <td>Same, and Oxygen-spectrum</td>
        <td class="nw vb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
        <td class="tdr vb"><a href="#plate18">154</a></td>
      </tr>
    </table>
  </section>
-->


  <!-- PART I_________________________________________________________ -->
  <section class="part">
    <h2 class="title-part" id="part-1">The Aurora and its characters</h2>
  </section>


  <!-- Chapitre 1_________________________________________________________-->
  <section class="chapter" id="chapter-1">

    <h3 class="titlechapter" id="chap-1">The aurora as known <br/>to the ancients </h3>
    <p class="shorter">The aurora as known to the ancients</p>


    <p>In Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv., we find the following:<span class="sidenote">Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv. Description of Auroræ.</span>—“Tempus est, alios quoque ignes percurrere,
      quorum diversæ figuræ sunt. Aliquando emicat stella, aliquando ardores sunt, aliquando fixi et hærentes,
      nonnunquam volubiles. Horum plura genera conspiciantur. Sunt <i>Bothynoë</i>&nbsp;<span class="footnote"><span class="greek">βόθυνος</span>, a hollow.</span> Seneca’s ‘Quæstiones Naturales,’ Lib. I. c. xiv. Description of Auroræ.</span>, quum velut corona cingente
      introrsus igneus cœli recessus est similis effossæ in orbem speluncæ. Sunt <i>Pithitæ</i>&nbsp;<span class="footnote"><span class="greek">πίθος</span>, a cask.</span>, quum magnitudo
      vasti rotundique ignis dolio similis, vel fertur vel in uno loco flagrat. Sunt <i>Chasmata</i>&nbsp;<span class="footnote"><span class="greek">χάσμς</span>, a chasm.</span>, quum aliquod
      cœli spatium desedit, et flammam dehiscens, velut in abdito, ostentat. Colores quoque omnium horum plurimi sunt. Quidam ruboris acerrimi, quidam evanidæ ac levis flammæ, quidam candidæ lucis, quidam micantes, quidam æqualiter et sine eruptionibus aut radiis fulvi.
    </p>

    <p class="separator"></p>

    <p><span class="sidenote">Seneca,c. xv.</span>C. xv. “Inter hæc ponas licet et quod frequenter in historiis legimus,
    cœlum ardere visum: cujus nonnunquam tam sublimis ardor est ut inter
    ipsa sidera videatur, nonnunquam tam humilis ut speciem longinqui incendii
    præbeat.</p>

    <p>“Sub Tiberio Cæsare cohortes in auxilium Ostiensis coloniæ cucurrerunt,
    tanquam conflagrantis, quum cœli ardor fuisset per magnam partem noctis,
    parum lucidus crassi fumidique ignis.”</p>

    <p>We may translate this:<span class="sidenote">Seneca,c. xv.</span>—“It is time other fires also to describe, of which
    there are diverse forms.</p>

    <p>“Sometimes a star shines forth; at times there are fire-glows, sometimes
    fixed and persistent, sometimes flitting. Of these many sorts may be distinguished.
    There are Bothynoë, when, as within a surrounding corona, the
    fiery recess of the sky is like to a cave dug out of space. There are Pithitæ,
    when the expanse of a vast and rounded fire similar to a tub (dolium) is either
    carried about or glows in one spot.</p>

    <p>“There are Chasmata, when a certain portion of the sky opens, and gaping
    displays the flame as in a porch. The colours also of all these are many.
    Certain are of the brightest red, some of a flitting and light flame-colour,
    some of a white light, others shining, some steadily and yellow without
    eruptions or rays.</p>

    <p class="separator"></p>


    <p>“Amongst these we may notice, what we frequently read of in history, the
    sky is seen to burn, the glow of which is occasionally so high that it may be
    seen amongst the stars themselves, sometimes so near the Earth (humilis) that
    it assumes the form of a distant fire. Under Tiberius Cæsar the cohorts ran
    together in aid of the colony of Ostia as if it were in flames, when the
    glowing of the sky lasted through a great part of the night, shining dimly
    like a vast and smoking fire.”</p>


    <p>From the above passages many striking particulars of the Aurora may be
    gathered; and by the division of the forms of Aurora into classes it is
    evident they were, at that period, the subject of frequent observation.
    The expression <span class="sidenote"> Auroræ frequently read of in history.</span>“et quod frequenter in historiis legimus” shows, too, that the
    phenomena of Auroral displays were a matter of record and discussion with
    the writers of the day.</p>

    <p>Various forms of Aurora may be recognized in the passages from Chap. xiv.;
    while in those from Chap. xv. a careful distinction is drawn between the
    Auroræ seen in the zenith or the upper regions of the sky, and those seen on
    the horizon or apparently (and no doubt in some cases actually) near the
    Earth’s surface.</p>


    <p>The description of the cohorts running to the fire only to find it an Aurora,
    calls to mind the many similar events happening in our own days. Not,
    however, but that a mistake may sometimes occur in an opposite direction.
    <span class="sidenote">A spurious Aurora.</span>“In the memoirs of Baron Stockmar an amusing anecdote is related of one
    Herr von Radowitz, who was given to making the most of easily picked up
    information. A friend of the Baron’s went to an evening party near Frankfort,
    where he expected to meet Herr von Radowitz. On his way he saw a
    barn burning, stopped his carriage, assisted the people, and waited till the
    flames were nearly extinguished. When he arrived at his friend’s house he
    found Herr von Radowitz, who had previously taken the party to the top of
    the building to see an Aurora, dilating on terrestrial magnetism, electricity,
    and so forth. Radowitz asked Stockmar’s friend, “Have you seen the beautiful
    Aurora Borealis?” He replied, “Certainly; I was there myself; it will soon
    be over.” An explanation followed as to the barn on fire: Radowitz was
    silent some ten minutes, then took up his hat and quietly disappeared.</p>



    <p>It is probable that many of the phantom combats which are recorded to
    have appeared in forms of fire in the air on the evenings preceding great
    battles might be traced to Auroræ, invested with distinct characteristics by
    the imagination of the beholders. <span class="sidenote">Auroræ as portents.</span>Auroræ are said to have appeared in the
    shape of armies of horse and foot engaged in battle in the sky before the
    death of Julius Cæsar, which they were supposed to foretell. For more than
    a year before the siege and destruction of Jerusalem by Titus Vespasian, the
    Aurora was said to have been frequently visible in Palestine.</p>

    <p>Josephus, in his ‘Wars of the Jews’ (Whiston’s Translation, Book VI.
    chap. v. sect. 3), in referring to the signs and wonders preceding the destruction
    of Jerusalem, speaks of a star or comet, and that a great light shone
    round about the altar and the holy house, which light lasted for half an hour,
    and that a few days after the feast of unleavened bread a certain prodigious
    and incredible phenomenon appeared—“for before sunsetting chariots and
    troops of soldiers in their armour were seen running about among the clouds,
    and surrounding of cities.” (This, if an Aurora, must have been an instance
    of a daylight one.)</p>

    <p>We find in Book II. of Maccabees, chap. v. verses 1, 2, 3, 4 (<span class="smcapuc">B.C.</span> about
    176 years):</p>


      <p>“1. About this same time Antiochus prepared his second voyage into Egypt:</p>

      <p>“2. And then it happened that through all the city, for the space almost of
      forty days, there were seen horsemen running in the air, in cloth of gold, and
      armed with lances like a band of soldiers.</p>

      <p>“3. And troops of horsemen in array, encountering and running one against
      another, with shaking of shields and multitude of pikes, and drawing of
      swords and casting of darts, and glittering of golden ornaments and harness
      of all sorts.</p>

      <p>“4. Wherefore every man prayed that that apparition might turn to
      good.”</p>



    <p><span class="sidenote">Early descriptions of Auroræ.</span>In Aristotle’s ‘De Meteoris,’ Lib. I. c. iv. and v., the Aurora is described
    as an appearance resembling flame mingled with smoke, and of a purple red
    or blue colour. Pliny (Lib. II. c. xxvii.) speaks of a bloody appearance of
    the heavens which seemed like a fire descending on the earth, seen in the
    third year of the 107th Olympiad, and of a light seen in the nighttime
    equal to the brightness of the day, in the Consulship of Cæcilius and Papirius
    (Lib. II. c. xxxiii.), both of which may be referred to Auroræ.</p>

    <p>In the ‘Annals of Philosophy,’ vol. ix. p. 250, it is stated that the Aurora
    among English writers is first described by Matthew of Westminster, who
    relates that in <span class="smcapuc">A.D.</span> 555 lances were seen in the air (“quasi species lancearum
    in aëre visæ sunt a septentrionali usque ad occidentem”).</p>

    <p>In the article in the ‘Edinb. Encyc.’ vol. iii. (1830), the Aurora (known to the
    vulgar as “streamers” or “merry dancers”) is distinguished in two kinds—the
    “tranquil” and the “varying.” Musschenbroek enumerates as forms:—<i>trabs</i>,
    “the beam,” an oblong tract parallel to the horizon; <i>sagitta</i>, “the
    arrow;” <i>faces</i>, “the torch;” <i>capra saltans</i>, “the dancing goat;” <i>bothynoë</i>,
    “the cave,” a luminous cloud having the appearance of a recess or hollow in
    the heavens, surrounded by a corona; <i>pithiæ</i>, “the tun,” an Aurora resembling
    a large luminous <i>cask</i>. The two sorts of Auroræ distinguished as
    the “bothynoë” and “pithiæ” are evidently taken from the passage in Seneca’s
    ‘Quæstiones’ before quoted. In ‘Liberti Fromondi Meteorologicorum’
    (London, 1656), Lib. II. cap. v. “De Meteoris supremæ regionis aëris,”
    art. 1. De Capra, Trabe, Pyramide, &amp;c., these and other fantastic forms
    attributable to Auroræ are more fully described.</p>

    <p>In the article “Aurora Polaris,” Encyc. Brit. edit. ix., we find noted that
    from a curious passage in Sirr’s ‘Ceylon and the Cingalese,’ vol. ii. p. 117, it
    would seem that the Aurora, or something like it, is visible occasionally in
    Ceylon, where the natives call it “Buddha Lights,” and that in many parts
    of Ireland a scarlet Aurora is supposed to be a shower of blood. The earliest
    mentioned Aurora (in Ireland) was in 688, in the ‘Annals of Cloon-mac-noise,’
    after a battle between Leinster and Munster, in which Foylcher O’Moyloyer
    was slain.</p>

    <p>In the article in the Edinb. Encyc. before referred to it is stated that it was
    not much more than a century ago that the phenomenon had been noticed to
    occur with frequency in our latitudes.</p>

    <p>Dr. Halley had begun to despair of seeing one till the fine display of
    1716.</p>

    <p><span class="sidenote">Early notices of Auroræ not frequent in our latitudes.</span> The first account on record in an English work is said to be in a book
    entitled ‘A Description of Meteors by W. F. D. D.’ (reprinted, London, 1654),
    which speaks of “burning spears” being seen January 30, 1560. The next
    is recorded by Stow as occurring on October 7, 1564; and, according to Stow
    and Camden, an Aurora was seen on two nights, 14th and 15th November,
    1574.</p>

    <p>Twice, again, an Aurora was seen in Brabant, 13th February and 28th
    September, 1575. Cornelius Gemma compared these to spears, fortified
    cities, and armies fighting in the air. Auroræ were seen in 1580 and 1581
    in Wirtemberg, Germany.</p>

    <p>Then we have no record till 1621, when an Aurora, described by Gassendi
    in his ‘Physics,’ was seen all over France, September 2nd of that year.</p>

    <p>In November 1623 another, described by Kepler, was seen all over
    Germany.</p>

    <p>From 1666 to 1716 no appearance is recorded in the ‘Transactions of the
    French Academy of Sciences;’ but in 1707 one was seen in Ireland and at
    Copenhagen; while in 1707 and 1708 the Aurora was seen five times.</p>

    <p>The Aurora of 1716, occurring after an interval of eighty years, which
    Dr. Halley describes, was very brilliant and extended over much country,
    being seen from the west of Ireland to the confines of Russia and the east of
    Poland, extending nearly 30° of longitude, and from about the 50th degree of
    latitude, over almost all the north of Europe, and in all places exhibiting at
    the same time appearances similar to those observed in London. An Aurora
    observed in Bologna in 1723 was stated to be the first that had ever been
    seen there; and one recorded in the ‘Berlin Miscellany’ for 1797 is called a
    very unusual phenomenon. Nor did Auroræ appear more frequent in the
    Polar Regions at that time, for Cælius states that the oldest inhabitants of
    Upsala considered the phenomenon as quite rare before 1716. Anderson, of
    Hamburg, writing about the same time, says that in Iceland the inhabitants
    themselves were astonished at the frequent Auroræ then beginning to take
    place; while Torfæus, the Icelander, who wrote in 1706, was old enough
    to remember the time when the Aurora was an object of terror in his native
    country.</p>

    <p>According to M. Mairan, 1441 Auroræ were observed between <span class="smcapuc">A.D.</span> 583
    and 1751, of which 972 were observed in the winter half-years and 469
    during the summer half-years. In our next Chapter we propose to give some
    general descriptions of Auroræ from comparatively early sources.</p>

  </section>

  <!-- Chapitre 2_________________________________________________________-->
  <section class="chapter" id="chapter-2">

    <h3 class="titlechapter" id="chap-2">Some general descriptions of auroræ</h3>
    <p class="shorter">Some general descriptions of auroræ</p>


    <p><span class="sidenote">Sir John Franklin’s description.</span>Sir John Franklin (‘Narrative of a Journey to the Shores of the Polar Sea
    in the years 1819, 1820, 1821, 1822’) describes an Aurora in these terms:—</p>

    <p><span class="sidenote">Parts of the Aurora: beams, flashes, and arches.</span>“For the sake of perspicuity I shall describe the several parts of the
    Aurora, which I term beams, flashes, and arches.</p>

    <p>“The beams are little conical pencils of light, ranged in parallel lines,
    with their pointed extremities towards the earth, generally in the direction
    of the dipping-needle.</p>

    <p><span class="sidenote">Formation of the Aurora.</span>“The flashes seem to be scattered beams approaching nearer to the earth,
    because they are similarly shaped and infinitely larger. I have called them
    flashes, because their appearance is sudden and seldom continues long.
    When the Aurora first becomes visible it is formed like a rainbow, the light
    of which is faint, and the motion of the beams undistinguishable. It is
    then in the horizon. As it approaches the zenith it resolves itself into
    beams which, by a quick undulating motion, project themselves into wreaths,
    afterwards fading away, and again and again brightening without any visible
    expansion or contraction of matter. Numerous flashes attend in different
    parts of the sky.”</p>

    <p><span class="sidenote">Arches of the Aurora.</span>Sir John Franklin then points out that this mass would appear like an arch
    to a person situated at the horizon by the rules of perspective, assuming its
    parts to be equidistant from the earth; and mentions a case when an Aurora,
    which filled the sky at Cumberland House from the northern horizon to the
    zenith with wreaths and flashes, assumed the shape of arches at some distance
    to the southward. He then continues:—“But the Aurora does not always
    make its first appearance as an arch. It sometimes rises from a confused mass
    of light in the east or west, and crosses the sky towards the opposite point,
    exhibiting wreaths of beams or coronæ boreales on its way. An arch also,
    which is pale and uniform at the horizon, passes the zenith without displaying
    any irregularity or additional brilliancy.” Sir John Franklin then mentions
    seeing three arches together, very near the northern horizon, one of which
    exhibited beams and even colours, but the other two were faint and uniform.
    (See example of a doubled arc Aurora observed at Kyle Akin, Skye, Plate
    VII.)</p>


    <p>He also mentions an arch visible to the southward exactly similar to one in
    the north. It appeared in fifteen minutes, and he suggests it probably had
    passed the zenith before sunset. The motion of the whole body of the
    Aurora from the northward to the southward was at angles not more than
    20° from the magnetic meridian. The centres of the arches were as often in
    the magnetic as in the true meridian. A delicate electrometer, suspended
    50 feet from the ground, was never perceptibly affected by the Aurora.</p>


    <p><span class="sidenote">Aurora does not often appear until some hours after sunset.</span>Sir John Franklin further remarks that the Aurora did not often appear
    immediately after sunset, and that the absence of that luminary for some
    hours was in general required for the production of a state of atmosphere
    favourable to the generation of the Aurora.</p>


    <p><span class="sidenote">Aurora seen in daylight.</span>On one occasion, however (March 8th, 1821), he observed it distinctly
    previous to the disappearance of daylight; and he subsequently states that
    on four occasions the coruscations of the Aurora were seen very distinctly
    before daylight had disappeared.</p>

    <p>[In the article “Aurora Polaris,” Encyc. Brit. edit. ix., the Transactions of
    the Royal Irish Academy, 1788, are referred to, where Dr. Usher notices that
    the Aurora makes the stars flutter in the telescope; and that, having remarked
    this effect strongly one day at 11 <span class="smcapuc">A.M.</span>, he examined the sky, and saw
    an Auroral corona with rays to the horizon.</p>

    <p>Instances are by no means rare of the principal Aurora-line having been
    seen in waning sunlight, and in anticipation of an Aurora which afterwards
    appeared.]</p>

    <span class="sidenote"> Auroral arch. Passage across the zenith.</span>

    <p><span class="sidenote">The Rev. James Farquharson’s observations.</span>The Rev. James Farquharson, from the observation of a number of Auroræ
    in Aberdeenshire in 1823 (‘Philosophical Transactions,’ 1829), concluded:—that
    the Aurora follows an invariable order in its appearance and progress;
    that the streamers appear first in the north, forming an arch from east to
    west, having its vertex at the line of the magnetic meridian <span class="sidenote">Auroral arch.</span>(when this
    arch is of low elevation it is of considerable breadth from north to south,
    having the streamers placed crosswise in relation to its own line, and all
    directed towards a point a little south of the zenith); that the arch moves
    forward towards the south, contracting laterally as it approaches the zenith,
    and increasing its intensity of light by the shortening of the streamers and the
    gradual shifting of the angles which the streamers near the east and west
    extremities of the arch make with its own line, till at length these streamers
    become parallel to that line, and then the arch is seen in a narrow belt 3° or
    4° only in breadth, stretching across the zenith at right angles to the magnetic
    meridian; that it still makes progress southwards, and after it has reached
    several degrees south of the zenith again enlarges its breadth by exhibiting
    an order of appearances the reverse of that which attended its progress
    towards the zenith from the north; <span class="sidenote">Passage across the zenith.</span>that the only conditions that can explain
    and reconcile these appearances are that the streamers of the Aurora are
    vertical, or nearly so, and form a deep fringe which stretches a great way from
    east to west at right angles to the magnetic meridian, but which is of no great
    thickness from north to south, and that the fringe moves southward, preserving
    its direction at right angles to the magnetic meridian.</p>


    <p><span class="sidenote">M. Lottin’s observations.</span>Dr. Lardner, in his ‘Museum of Science and Art,’ vol. x. p. 189 <i>et seq.</i>,
    alludes to a description of “this meteor” (<i>sic</i>) supplied by M. Lottin, an officer
    of the French Navy, and a Member of the Scientific Commission to the North
    Seas. Between September 1838 and April 1839, being the interval when
    the sun was constantly below the horizon, this savant observed nearly 150
    Auroræ. During this period sixty-four were visible, besides many concealed
    by a clouded sky, but the presence of which was indicated by the disturbances
    they produced upon the magnetic needle.</p>

    <p>The succession of appearances and changes presented by these “meteors”
    is thus graphically described by M. Lottin:—</p>

    <p><span class="sidenote">Formation of the auroral bow.</span>“Between four and eight o’clock <span class="smcapuc">P.M.</span> a light fog, rising to the altitude of
    six degrees, became coloured on its upper edge, being fringed with the light
    of the meteor rising behind it. This border, becoming gradually more
    regular, took the form of an arch, of a pale yellow colour, the edges of which
    were diffuse, the extremities resting on the horizon. This bow swelled slowly
    upwards, its vertex being constantly on the magnetic meridian. Blackish
    streaks divided regularly the luminous arc, and resolved it into a system of
    rays. These rays were alternately extended and contracted, sometimes slowly,
    sometimes instantaneously, sometimes they would dart out, increasing and
    diminishing suddenly in splendour. The inferior parts, or the feet of the
    rays, presented always the most vivid light, and formed an arc more or less
    regular. The length of these rays was very various, but they all converged
    to that point of the heavens indicated by the direction of the southern pole
    of the dipping-needle. Sometimes they were prolonged to the point where
    their directions intersected, and formed the summit of an enormous dome of
    light.</p>


    <p><span class="sidenote">It ascends to the zenith.</span>“The bow then would continue to ascend toward the zenith. It would
    suffer an undulatory motion in its light—that is to say, that from one extremity
    to the other the brightness of the rays would increase successively in
    intensity. This luminous current would appear several times in quick succession,
    and it would pass much more frequently from west to east than in
    the opposite direction. Sometimes, but rarely, a retrograde motion would
    take place immediately afterward; and as soon as this wave of light had run
    successively over all the rays of the Aurora from west to east, it would return
    in the contrary direction to the point of its departure, producing such an
    effect that it was impossible to say whether the rays themselves were actually
    affected by a motion of translation in a direction nearly horizontal, or if this
    more vivid light was transferred from ray to ray, the system of rays themselves
    suffering no change of position. The bow, thus presenting the appearance
    of an alternate motion in a direction nearly horizontal, had usually
    the appearance of the undulations or folds of a ribbon or flag agitated by the
    wind. Sometimes one, and sometimes both of its extremities would desert
    the horizon, and then its folds would become more numerous and marked,
    the bow would change its character and assume the form of a long sheet of
    rays returning into itself, and consisting of several parts forming graceful
    curves. The brightness of the rays would vary suddenly, sometimes surpassing
    in splendour stars of the first magnitude; these rays would rapidly
    dart out, and curves would be formed and developed like the folds of a
    serpent; then the rays would affect various colours, the base would be red,
    the middle green, and the remainder would preserve its clear yellow hue.
    Such was the arrangement which the colours always preserved. They were
    of admirable transparency, the base exhibiting blood-red, and the green of
    the middle being that of the pale emerald; the brightness would diminish,
    the colours disappear and all be extinguished, sometimes suddenly and sometimes
    by slow degrees. After this disappearance fragments of the bow
    would be reproduced, would continue their upward movement and approach
    the zenith; the rays, by the effect of perspective, would be gradually
    shortened; the thickness of the arc, which presented then the appearance of
    a large zone of parallel rays, would be extended; then the vertex of the
    bow would reach the magnetic zenith, or the point to which the south pole
    of the dipping-needle is directed. <span class="sidenote">Reaches the zenith.</span>At that moment the rays would be seen
    in the direction of their feet. If they were coloured they would appear as
    a large red band, through which the green tints of their superior parts could
    be distinguished, and if the wave of light above mentioned passed along
    them their feet would form a long sinuous undulating zone; while throughout
    all these changes the rays would never suffer any oscillation in the direction
    of their axis, and would constantly preserve their mutual parallelisms.</p>

    <p>“While these appearances are manifested new bows are formed, either
    commencing in the same diffuse manner or with vivid and ready formed rays;
    they succeed each other, passing through nearly the same phases, and arrange
    themselves at certain distances from each other. <span class="sidenote">Multiple bows.</span>As many as nine have been
    counted having their ends supported on the earth, and in their arrangement
    resembling the short curtains suspended one behind the other over the scene
    of a theatre, and intended to represent the sky. Sometimes the intervals
    between these bows diminish, and two or more of them close upon each
    other, forming one large zone traversing the heavens and disappearing towards
    the south, becoming rapidly feeble after passing the zenith. But sometimes
    also, when this zone extends over the summit of the firmament from east to
    west, <span class="sidenote">Corona formed.</span>the mass of rays appear suddenly to come from the south, and to form,
    with those from the north, the real boreal corona, all the rays of which
    converge to the zenith. This appearance of a crown, therefore, is doubtless
    the mere effect of perspective; and an observer placed at the same instant at
    a certain distance to the north or to the south would perceive only an arc.</p>

    <p>“The total zone, measuring less in the direction north and south than in
    the direction east and west, since it often leans upon the corona, would be
    expected to have an elliptical form; but that does not always happen: it has
    been seen circular, the unequal rays not extending to a greater distance than
    from eight to twelve degrees from the zenith, while at other times they reach
    the horizon.</p>

    <p>“Let it then be imagined that all these vivid rays of light issue forth
    with splendour, subject to continual and sudden variations in their length
    and brightness; that these beautiful red and green tints colour them at intervals;
    that waves of light undulate over them; that currents of light
    succeed each other; and in fine, that the vast firmament presents one immense
    and magnificent dome of light, reposing on the snow-covered base supplied
    by the ground, which itself serves as a dazzling frame for a sea calm and
    black as a pitchy lake. And some idea, though an imperfect one, may be
    obtained of the splendid spectacle which presents itself to him who witnesses
    the Aurora from the Bay of Alten.</p>


    <p><span class="sidenote">Duration of corona.</span>“The corona when it is formed only lasts for some minutes; it sometimes
    forms suddenly, without any previous bow. There are rarely more than two
    on the same night, and many of the Auroras are attended with no crown
    at all.</p>



    <p>“The corona becomes gradually faint, the whole phenomenon being to the
    south of the zenith, forming bows gradually paler and generally disappearing
    before they reach the southern horizon. All this most commonly takes place
    in the first half of the night, after which the Aurora appears to have lost its
    intensity; the pencils of rays, the bands, and the fragments of bows appear
    and disappear at intervals. Then the rays become more and more diffused,
    and ultimately merge into the vague and feeble light which is spread over
    the heavens, grouped like little clouds, and designated by the name of
    auroral plates (plaques aurorales). Their milky light frequently undergoes
    striking changes in the brightness, like motions of dilatation and contraction,
    which are propagated reciprocally between the centre and the circumference,
    like those which are observed in marine animals called Medusæ. <span class="sidenote">Disappearance of Aurora.</span>The phenomena
    become gradually more faint, and generally disappear altogether on
    the appearance of twilight. Sometimes, however, the Aurora continues
    after the commencement of daybreak, when the light is so strong that a
    printed book may be read. It then disappears, sometimes suddenly; but it
    often happens that, as the daylight augments, the Aurora becomes gradually
    vague and undefined, takes a whitish colour, and is ultimately so mingled
    with the cirro-stratus clouds that it is impossible to distinguish it from them.”</p>

    <p>Lieutenant Weyprecht has grandly described forms of Aurora in Payer’s
    ‘New Lands within the Arctic Circle’ (vol. i. p. 328 <i>et seq.</i>) as follows:—</p>


    <p><span class="sidenote">Lieut. Weyprecht’s description.</span>“There in the south, low on the horizon, stands a faint arch of light. It
    looks as it were the upper limit of a dark segment of a circle; but the stars,
    which shine through it in undiminished brilliancy, convince us that the
    darkness of the segment is a delusion produced by contrast. Gradually the
    arch of light grows in intensity and rises to the zenith. It is perfectly
    regular; its two ends almost touch the horizon, and advance to the east and
    west in proportion as the arch rises. No beams are to be discovered in it,
    but the whole consists of an almost uniform light of a delicious tender colour.
    It is transparent white with a shade of light green, not unlike the pale green
    of a young plant which germinates in the dark. The light of the moon
    appears yellow contrasted with this tender colour, so pleasing to the eye and
    so indescribable in words, a colour which nature appears to have given only
    to the Polar Regions by way of compensation. The arch is broad, thrice the
    breadth, perhaps, of the rainbow, and its distinctly marked edges are strongly
    defined on the profound darkness of the Arctic heavens. The stars shine
    through it with undiminished brilliancy. The arch mounts higher and
    higher. An air of repose seems spread over the whole phenomenon; here
    and there only a wave of light rolls slowly from one side to the other. It
    begins to grow clear over the ice; some of its groups are discernible. The
    arch is still distant from the zenith, a second detaches itself from the dark
    segment, and this is gradually succeeded by others. All now rise towards
    the zenith; the first passes beyond it, then sinks slowly towards the northern
    horizon, and as it sinks loses its intensity. <span class="sidenote">Formation of arches.</span>Arches of light are now stretched
    over the whole heavens; seven are apparent at the same time on the sky,
    though of inferior intensity. The lower they sink towards the north the
    paler they grow, till at last they utterly fade away. Often they all return
    over the zenith, and become extinct just as they came.</p>


    <p>“It is seldom, however, that an Aurora runs a course so calm and so
    regular. The typical dark segment, which we see in treatises on the subject,
    in most cases does not exist. A thin bank of clouds lies on the horizon.
    <span class="sidenote">Band of light appears.</span>The upper edge is illuminated; out of it is developed a band of light, which
    expands, increases in intensity of colour, and rises to the zenith. The colour
    is the same as in the arch, but the intensity of the colour is stronger. The
    colours of the band change in a never-ceasing play, but place and form
    remain unaltered. The band is broad, and its intense pale green stands out
    with wonderful beauty on the dark background. Now the band is twisted
    into many convolutions, but the innermost folds are still to be seen distinctly
    through the others. Waves of light continually undulate rapidly through
    its whole extent, sometimes from right to left, sometimes from left to right.
    Then, again, it rolls itself up in graceful folds. It seems almost as if breezes
    high in the air played and sported with the broad flaming streamers, the
    ends of which are lost far off on the horizon. The light grows in intensity,
    the waves of light follow each other more rapidly, prismatic colours appear
    on the upper and lower edge of the band, the brilliant white of the centre
    is enclosed between narrow stripes of red and green. Out of one band have
    now grown two. <span class="sidenote"> Second band and rays.</span>The upper continually approaches the zenith, rays begin
    to shoot forth from it towards a point near the zenith to which the south
    pole of the magnetic needle, freely suspended, points.</p>

    <p>“The band has nearly reached it, and now begins a brilliant play of rays
    lasting for a short time, the central point of which is the magnetic pole—a
    sign of the intimate connexion of the whole phenomenon with the magnetic
    forces of the earth. Round the magnetic pole short rays flash and flare
    on all sides, prismatic colours are discernible on all their edges, longer and
    shorter rays alternate with each other, waves of light roll round it as a centre.
    <span class="sidenote">Corona formed.</span>What we see is the auroral corona, and it is almost always seen when a band
    passes over the magnetic pole. This peculiar phenomenon lasts but a short
    time. The band now lies on the northern side of the firmament, gradually
    it sinks, and pales as it sinks; it returns again to the south to change and
    play as before. So it goes on for hours, the Aurora incessantly changes
    place, form, and intensity. It often entirely disappears for a short time,
    only to appear again suddenly, without the observers clearly perceiving how
    it came and where it went; simply, it is there.</p>

    <p><span class="sidenote">Single-rayed band.</span>“But the band is often seen in a perfectly different form. Frequently it
    consists of single rays, which, standing close together, point in an almost
    parallel direction towards the magnetic pole. These become more intensely
    bright with each successive wave of light; hence each ray appears to flash
    and dart continually, and their green and red edges dance up and down as
    the waves of light run through them. Often, again, the rays extend through
    the whole length of the band, and reach almost up to the magnetic pole.
    These are sharply marked, but lighter in colour than the band itself, and in
    this particular form they are at some distance from each other. Their colour
    is yellow, and it seems as if thousands of slender threads of gold were
    stretched across the firmament. A glorious veil of transparent light is spread
    over the starry heavens; the threads of light with which this veil is woven
    are distinctly marked on the dark background; its lower border is a broad
    intensely white band, edged with green and red, which twists and turns in
    constant motion. A violet-coloured auroral vapour is often seen simultaneously
    on different parts of the sky.</p>



    <p><span class="sidenote">Aurora in stormy weather.</span>“Or, again, there has been tempestuous weather, and it is now, let us
    suppose, passing away. Below, on the ice, the wind has fallen; but the
    clouds are still driving rapidly across the sky, so that in the upper regions its
    force is not yet laid. Over the ice it becomes somewhat clear; behind the
    clouds appears an Aurora amid the darkness of the night. Stars twinkle
    here and there; through the opening of the clouds we see the dark firmament,
    and the rays of the Aurora chasing one another towards the zenith. The
    heavy clouds disperse, mist-like masses drive on before the wind. <span class="sidenote">Fragments.</span>Fragments
    of the northern lights are strewn on every side: it seems as if the storm had
    torn the Aurora bands to tatters, and was driving them hither and thither
    across the sky. These threads change form and place with incredible
    rapidity. Here is one! lo, it is gone! Scarcely has it vanished before it
    appears again in another place. Through these fragments drive the waves of
    light: one moment they are scarcely visible, in the next they shine with
    intense brilliancy. But their light is no longer that glorious pale green; it
    is a dull yellow. It is often difficult to distinguish what is Aurora and what
    is vapour; the illuminated mists as they fly past are scarcely distinguishable
    from the auroral vapour which comes and goes on every side.</p>




    <p><span class="sidenote">Bands.</span>“But, again, another form. Bands of every possible form and intensity
    have been driving over the heavens. It is now eight o’clock at night, the
    hour of the greatest intensity of the northern lights. For a moment some
    bundles of rays only are to be seen in the sky. In the south a faint, scarcely
    visible band lies close to the horizon. All at once it rises rapidly, and spreads
    east and west. The waves of light begin to dart and shoot, some rays mount
    towards the zenith. For a short time it remains stationary, then suddenly
    springs to life. The waves of light drive violently from east to west, the
    edges assume a deep red and green colour, and dance up and down. The
    rays shoot up more rapidly, they become shorter; all rise together and
    approach nearer and nearer to the magnetic pole. <span class="sidenote">Rays reach the pole.</span>It looks as if there were
    a race among the rays, and that each aspired to reach the pole first. And
    now the point is reached, and they shoot out on every side, to the north and
    the south, to the east and the west. Do the rays shoot from above downwards,
    or from below upwards? Who can distinguish? From the centre
    issues a sea of flames: is that sea red, white, or green? Who can say? It is
    all three colours at the same moment! The rays reach almost to the horizon:
    the whole sky is in flames. Nature displays before us such an exhibition of
    fireworks as transcends the powers of imagination to conceive. Involuntarily
    we listen; such a spectacle must, we think, be accompanied with sound.
    <span class="sidenote">No noise.</span>But unbroken stillness prevails; not the least sound strikes on the ear.
    Once more it becomes clear over the ice, and the whole phenomenon has
    disappeared with the same inconceivable rapidity with which it came, and
    gloomy night has again stretched her dark veil over everything. This was the
    Aurora of the coming storm—the Aurora in its fullest splendour. No pencil
    can draw it, no colours can paint it, and no words can describe it in all its
    magnificence.”</p>

    <p>A reproduction of the woodcut in Payer’s ‘Austrian Arctic Voyages,’
    illustrating some of the features of the above description, will be found on
    <span class="cross-ref">Plate I</span>.</p>

    <figure class="plate" id="plate1">
      <!-- <img src="assets/aurorae/images/plate1.jpg" /> -->
      <figcaption>The aurora during the ice-pressure.</figcaption>
      <p class="figsource"><i>[Payer's Austrian Artics Voyages.]</i></p>
    </figure>

    <p>In the ‘Edinburgh Encyclopædia,’ article “Aurora,” we find:—</p>



    <p><span class="sidenote">Descriptions of Auroræ in high Northern latitudes.</span>“In high Northern latitudes the Auroræ Boreales are singularly resplendent,
    and even terrific.</p>

    <p>“They frequently occupy the whole heavens, and, according to the testimony
    of some, eclipse the splendour of stars, planets, and moon, and even of the
    sun itself.</p>

    <p><span class="sidenote">In Siberia.</span>“In the south-eastern districts of Siberia, according to the description of
    Gmelin, cited and translated by Dr. Blagden (Phil. Trans. vol. lxxiv. p. 228),
    the Aurora is described to begin with single bright pillars, rising in the
    north, and almost at the same time in the north-east, which, gradually
    increasing, comprehend a large space of the heavens, rush about from place
    to place with incredible velocity, and finally almost cover the whole sky up
    to the zenith, and produce an appearance as if a vast tent were expanded in
    the heavens, glittering with gold, rubies, and sapphires. A more beautiful
    spectacle cannot be painted; but whoever should see such a northern light
    for the first time could not behold it without terror.”</p>


    <p><span class="sidenote">Maupertius’s description at Oswer-Zornea.</span>Maupertius describes a remarkable Aurora he saw at Oswer-Zornea on the
    18th December, 1876. An extensive region of the heavens towards the
    south appeared tinged of so lively a red that the whole constellation of Orion
    seemed as if dyed in blood. The light was for some time fixed, but soon
    became movable, and, after having successively assumed all the tints of violet
    and blue, it formed a dome of which the summit nearly approached the
    zenith in the south-west.</p>

    <p><span class="sidenote">Red Auroræ rare in Lapland.</span>Maupertius adds that he observed only two of the red northern lights in
    Lapland, and that they are of very rare occurrence in that country.</p>

    <p>The observations of Carl Bock, the Norwegian naturalist, kindly communicated
    by him to me, and detailed in Chapter III., quite confirm this
    observation of Maupertius as to the rare occurrence of red Auroræ in
    Lapland, he having only seen one.</p>

  </section>

  <!-- Chapitre 3_________________________________________________________-->
  <section class="chapter" id="chapter-3">
    <h3 class="titlechapter" id="chap-3">Some specific descriptions of auroræ, including results of the english arctic expedition, 1875-76</h3>
    <p class="shorter">Some specific descriptions of auroræ</p>

    <h4 id="chap-3-1">Captain Sabine’s Auroræ.</h4>

    <p>Captain Sabine describes Auroræ seen at Melville Island<span class="sidenote">Captain Sabine’s Auroræ.</span>(Parry’s first
    voyage, January 15). Towards the southern horizon an ordinary Aurora
    appeared. The luminous arch broke into masses streaming in different
    directions, always to the east of the zenith.</p>


    <p>The various masses seemed to arrange themselves in two arches, one
    passing near the zenith and a second midway between the zenith and the
    horizon<span class="sidenote">Curvature of arches towards each other.</span>, both north and south, but curving towards each other. At one
    time a part of the arch near the zenith was bent into convolutions like a
    snake in motion and undulating rapidly.</p>

    <h4 id="chap-3-2">Aurora seen at Sunderland, February 8th, 1817.</h4>

    <p>(‘Annals of Philosophy,’ vol. ix. p. 250.)</p>

    <p>It began about 7 <span class="smcapuc">P.M.</span> during a strong gale from the N.W., with single
    bright streamers in the N. and N.W., which covered a large space and rushed
    about from place to place with amazing velocity, and had a fine tremulous
    motion, illuminating the hemisphere as much as the moon does eight or nine
    days from change. About 11 o’clock part of the streamers appeared as if
    projected south of the zenith and looked like the pillars of an immense
    amphitheatre<span class="sidenote">Aurora seen at Sunderland, Feb. 8, 1817. Formation of corona.</span>, presenting a most brilliant spectacle and seeming to be in a
    lower region of the atmosphere, and to descend and ascend in the air for
    several minutes. (This appears to have been the formation of a corona.)
    One streamer passed over Orion, but neither increased nor diminished its
    splendour.</p>

    <h4 id="chap-3-3">Description of Aurora by Dr. Hayes, 6th January, 1861.</i></h4>



    <p><span class="sidenote">Dr. Hayes’s Aurora, 6th January, 1861.</span>‘Recent Polar Voyages’ contains a narrative of the voyage of Dr. Hayes,
    who sailed from Boston on the 6th of July, 1860, and wintered at Port
    Foulbe. He witnessed a remarkable display of the Aurora Borealis on the
    morning of the 6th January, 1861.<span class="sidenote">Development of Aurora.</span></p>




    <p>The darkness was so profound as to be oppressive. Suddenly, from the
    rear of the black cloud which obscured the horizon, flashed a bright ray.
    Presently an arch of many colours fixed itself across the sky, and the Aurora
    gradually developed.</p>


    <p>The space within the arch was filled by the black cloud; but its borders
    brightened steadily, though the rays discharged from it were exceeding
    capricious, now glaring like a vast conflagration, now beaming like the glow
    of a summer morn. More and more intense grew the light, until, from
    irregular bursts, it matured into an almost uniform sheet of radiance.
    Towards the end of the display its character changed. Lurid fires flung
    their awful portents across the sky, before which the stars seemed to recede
    and grow pale.</p>


    <p>The colour of the light was chiefly red; but every tint had its turn, and
    sometimes two or three were mingled; blue and yellow streamers shot across
    the terrible glare, or, starting side by side from the wide expanse of the
    radiant arch, melted into each other, and flung a strange shade of emerald
    over the illuminated landscape. Again this green subdues and overcomes
    the red; then azure and orange blend in rapid flight, subtle rays of violet
    pierce through a broad flash of yellow, and the combined streams issue in
    innumerable tongues of white flame, which mount towards the zenith. <span class="sidenote">Mixed colours. Colours change. Tongues of white flame formed.</span>
    </p>

    <p>The illustration which accompanies this description in the work is reproduced
    on Plate II., and forcibly reminds one of the “curtains” of the
    Aurora described in the preceding Chapter by Mons. Lottin.</p>



    <figure class="plate" id="plate2">
      <!-- <img src="assets/aurorae/images/plate2.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-4">Prof. Lemström’s Aurora of 1st September, 1868.</h4>



    <p><span class="sidenote">Prof. Lemström’s Aurora, 1st September, 1868.</span>In the first Swedish Expedition, 1868, some remarkable observations were
    made on the appearance of luminous beams around the tops of mountains,
    which M. Lemström showed by the spectroscope to be of the same nature as
    Auroræ.</p>



    <p>On the 1st September, 1868, on the Isle of Amsterdam in the Bay of
    Sweerenberg, there was a light fall of snow, and the snowflakes were observed
    falling obliquely. <span class="sidenote">Aurora from earth’s surface.</span>All at once there appeared a luminous phenomenon
    which, starting from the earth’s surface, shot up vertically, cutting the
    direction of the falling snowflakes, and this appearance lasted for some
    seconds.  <span class="sidenote">Yellow-green line seen.</span>On examination with a spectroscope the yellow-green line was
    found by Lemström (but of feeble intensity) when the slit of the instrument
    was directed towards a roof or other object covered with snow, and even in
    the snow all round the observer.</p>




    <p><span class="sidenote">Lemström’s conclusions.</span>M. Lemström concluded that an electric discharge of an auroral nature,
    which could only be detected by means of the spectroscope, was taking place
    on the surface of the ground all around him, and that, from a distance, it
    would appear as a faint display of Aurora.</p>

    <p>[It should, however, here be noted that the reflection of an Aurora from a
    white or bright surface would give, in a fainter degree, the spectrum of the
    Aurora itself; and, apart from the phenomena seen by the eye, the case
    fails to be conclusive that an Aurora on the surface of the ground was
    examined.]</p>

    <h4 id="chap-3-5">Mr. J. R. Capron’s Aurora of October 24th, 1870.</h4>






    <p><span class="sidenote">Mr. J. R. Capron’s Aurora, Oct. 24, 1870.</span>The description, from my notes made at the time of this fine display, is as
    follows:—“Last evening (October 24) the Aurora Borealis was again most
    beautifully seen here (Guildford). At 6 <span class="smcapuc">P.M.</span> indications of the coming
    display were visible in the shape of <span class="sidenote">Silver glow in north.</span>a bright silver glow in the north, which
    contrasted strongly with the opposite dark horizon. For two hours this
    continued, with the addition from time to time of a crimson glow in the
    north-east, <span class="sidenote">Phosphorescent cloud-streamers.</span>and of streamers of opaque-white phosphorescent cloud, shaped
    like horse-tails (very different from the more common transparent auroral
    diverging streams of light), which floated upwards and across the sky from
    east and west to the zenith. At about 8 o’clock the display culminated; and
    few observers, I should think, ever saw a more lovely sky-picture.
    <span class="sidenote">Crimson masses on horizon.</span>Two
    patches of intense crimson light about this time massed themselves on the
    north-east and north-west horizon, the sky between having a bright silver
    glow. The crimson masses became more attenuated as they mounted upwards;
    and from them there suddenly ran up bars or streamers of crimson
    and gold light,<span class="sidenote">Coloured streamers.</span> which, as they rose, curved towards each other in the north,
    and, ultimately meeting, formed a glorious arch of coloured light, having at
    its apex an oval white luminous corona or cloud of similar character to the
    phosphorescent clouds previously described, but brighter. <span class="sidenote">Corona formed.</span>At this time the
    spectator appeared to be looking at the one side of a cage composed of
    glowing red and gold bars, which extended from the distant parts of the
    horizon to a point over his head. Shortly after this the Auroral display
    gradually faded away, and at 9 o’clock the sky was of its usual appearance,
    <span class="sidenote">Aurora fades away.</span>except that the ordinary tint seemed to have more of indigo, probably by
    contrast with the marvellous colours which had so lately shone upon it.”</p>




    <span class="sidenote">T. F.’s description of same at Torquay.</span><p>T. F., describing the same Aurora from Torquay, says it showed itself at
    sundown, attained its maximum at 8, and lasted until 11. At 8 o’clock more
    than half the visible heavens was one sea of colour; the general ground
    greenish yellow and pale rose, with extensive shoals of deep rose in the east
    and west; while from the north, streaming upwards to and beyond the
    zenith, were tongues and brushes of rosy red, so deep that the sky between
    looked black. Mr. Gibbs reported that in London, at about 8 o’clock, <span class="sidenote">TMr. Gibbs’s report in London.</span>brilliant
    crimson rays shot up to the zenith, and the sky seemed one mass of fire.</p>

    <p>A facsimile of my water-colour sketch of this fine discharge is given on
    Plate III.</p>



    <figure class="plate" id="plate3">
      <!-- <img src="assets/aurorae/images/plate3.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-6">Mr. Barker’s (superposed) red and white Auroræ, 9th November (1870?).</h4>





    <p><span class="sidenote">Mr. Barker’s Auroræ, 9th November (1870?).</span>On the 9th November (1870?) Mr. Barker saw at New Haven (U. S.) a most
    magnificent crimson Aurora. At about a quarter to 6 <span class="smcapuc">P.M.</span> it consisted of a
    brilliant streamer shooting up from the north-western horizon. This was
    continued in a brilliant red<span class="sidenote">Red Aurora.</span>, but rather nebulous, mass of light passing
    upwards and to the north. Its highest points were from 30° to 40° in
    altitude. <span class="sidenote">White Aurora.</span>A white Aurora, consisting of bright streamers, appeared simultaneously
    and extended round to the north-east. Prof. Newton informed
    Mr. Barker that he had observed an equally brilliant red patch of auroral
    light in the north-east five or ten minutes earlier.</p>



    <p><span class="sidenote">Red seen through white.</span>Since the lower end of the red streamers was much lower than that of the
    white, it would seem as if the red were seen through the white, the red being
    most remote.</p>



    <p><span class="sidenote">Crimson line not seen in white Aurora.</span>Spectroscopic observations of this Aurora were made. The crimson Aurora
    lasted less than half an hour, and then disappeared. In the white Aurora,
    which remained, the crimson line could not be seen.</p>



    <span class="sidenote">Carl Bock’s vibrating rays.</span><p>It may be here noted that during the Aurora seen by Carl Bock in Lapland,
    and painted by him by its own light (described, p. 25), he had the impression
    of sets of vibrating rays behind each other, and in the drawing it looks as if
    streamers were seen behind an arc.</p>

    <figure class="plate" id="plate4">
      <!-- <img src="assets/aurorae/images/plate4.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-7">Mr. J. R. Capron’s Aurora of February 4th, 1872.</h4>


    <p><span class="sidenote">Mr. J. R. Capron’s Aurora, Feb. 4, 1872. Masses of phosphorescent vapour. Rose tints appeared. Aurora from behind clouds. Formation of corona. Duration of corona. Streamers from corona. Rain during Aurora. Wind during night. Phosphorescent clouds preceded the Aurora in daylight.</span>
My description of this Aurora as seen at Guildford, and as given at the time,
    is as follows:—“Last evening, returning from church a little before 8 <span class="smcapuc">P.M.</span>,
    the sky presented a weird and unusual aspect, which at once struck the eye.
    A lurid tinge upon the clouds which hung around suggested the reflection of
    a distant fire; while scattered among these, torn and broken masses of vapour,
    having a white and phosphorescent appearance, and quickly changing their
    forms, reminded me of a similar appearance preceding the great Aurora of
    24th October, 1870. Shortly some of these shining white clouds or vapours
    partly arranged themselves in columns from east to west, and at the same
    time appeared the characteristic patches of rose-coloured light which are
    often seen in an auroral display. About 8 o’clock the clouds had to a certain
    extent broken away, and the Aurora shone out from behind heavy banks of
    vapour, which still rested on the eastern horizon, the north-west horizon
    being free from cloud and glowing brightly with red light. And now, at
    about 8.15, was presented a most beautiful phenomenon. While looking
    upwards, I saw a corona or stellar-shaped mass of white light form in the
    clear blue sky immediately above my head&nbsp;<span class="footnote">M. Lemström (Swedish Expedition, 1868) concludes that the corona of the Aurora Borealis
is not entirely a phenomenon of perspective, but that the rays have a true curvature, that they
    are currents flowing in the same direction and attract each other. There is also an account
    [<i>antè</i>, p. 16] of an Aurora at Melville Island (Parry’s first voyage), in which two arches were
    seen curving towards each other.</span>, not by small clouds or rays
    collecting, but more in the way that a cloud suddenly forms by condensation
    in the clear sky on a mountain top, or a crystal shoots in a transparent
    liquid, having too, as I thought, an almost traceable nucleus or centre, from
    which spear-like rays projected. From this corona in a few seconds shot
    forth diverging streamers of golden light, which descended to and mingled
    with the rosy patches of the Aurora hanging about the horizon. The spaces
    of sky between the streamers were of a deep purple (probably an effect of
    contrast). The display of the corona, though lasting a few minutes only,
    was equal to, if not excelling in beauty, the grand display of October 1870,
    before described, in which case, however, a ring or disk of white light of considerable
    size took the place of the stellar-shaped corona. What struck me
    particularly was the corona developing itself as from a centre in the clear
    sky, and the diverging streamers apparently shooting downwards, whereas in
    general the streamers are seen to shoot up from the horizon and converge
    overhead. The effect may have been an illusion; but, if so, it was a remarkable
    one. The general Aurora lasted for some time, till it was lost in a
    clouded sky; and, in fact, rain was descending at one time while the Aurora
    was quite bright. Strong wind prevailed during the night&nbsp;<span class="footnote">A brilliant display in December 1870, on the east coast of Sicily, was followed by very
    violent storms, with the overflow of the Tiber and the flooding of Rome.</span>. The Aurora
    was probably very extensive, as the evening, notwithstanding the clouds, was
    nearly as bright as moonlight. The peculiar clouds referred to must have
    preceded the Aurora in daylight, as I recollect seeing them at 6.30 as we
    went to church.”</p>

    <span class="sidenote">Aurora
    predicted.</span>

    <p>They had even then a peculiarly wild, ragged, and phosphorescent appearance,
    and so much resembled some I had seen to accompany the Aurora
    of October 1870, that I predicted (as came to pass) a display later in the
    evening. A <i>facsimile</i> of my water-colour sketch of this Aurora is given on
    Plate IV. fig. 1, while the corona and rays are represented (with rather too
    hard an outline) on Plate V. fig. 2.</p>

    <figure class="plate" id="plate5">
      <!-- <img src="assets/aurorae/images/plate5.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-8"><i>Description of an Aurora seen at Cardiff.</i></h4>

    <span class="sidenote">Aurora seen
    at Cardiff. Formation
    of corona.</span>

    <p>An Aurora was seen at Cardiff. A dusky red aspect of the sky towards the
    north, and extending itself across the zenith westward, made its appearance
    about half-past 5 <span class="smcapuc">P.M.</span> The lights reached their greatest intensity at 6 o’clock,
    when the sky was suffused with a rich crimson glow, a broad band of colour
    reaching from N.E. to W. A corona of deep hue, having rugged sharply
    defined edges, stood out prominently in the zenith, apparently on a parallel
    plane to the earth, and having its centre almost immediately over the head of
    the spectator.</p>

    <span class="sidenote">Radii
    thrown out
    from corona.</span>

    <p>From this corona, elliptic in form, and in its broadest diameter about four
    times the size of the moon, there were thrown out brilliant silvery blue radii,
    extending to the N.E. and W. horizon, and presenting the appearance of a
    vast cupola of fire.</p>

    <span class="sidenote">Rain fell
    when Aurora
    died out.</span>

    <p>At half-past 6 the lights died completely out, leaving masses of cloud
    drifting up from the south, and a shower of rain fell. The corona was
    remarked upon as unusual. At Edinburgh the sky was brilliant for several
    hours. (The date of this Aurora is uncertain, as the account is from an
    undated newspaper cutting. It is supposed to be in February 1872, but
    could hardly have been on the 4th, as the Aurora of that date did not reach
    its maximum development at Edinburgh till 8 <span class="smcapuc">P.M.</span>)</p>

    <h4 id="chap-3-9"><i>Mr. J. R. Capron’s Aurora, seen at Guildown, Guildford,
    February 4th, 1874.</i></h4>

    <span class="sidenote">Silvery
    brightness
    in N.E. Light-cloud,
    which moved
    from E. to
    W. Formation
    of arc in N. Streamers. Horizontal
    clouds of
    misty light.</span>

    <p>About 7 <span class="smcapuc">P.M.</span> my attention was drawn to a silvery brightness in the north-east.
    Above, and still more to the east, was a bright cloud of light, which
    looked dense and misty, and gave one the impression of an illuminated fog-cloud.
    The edges were so bright that the adjacent sky, but for the stars
    shining in it, might, by contrast, have been taken for a dark storm-cloud.
    The light-cloud expanded upwards until its apex became conical, and then
    moved rapidly from east, along the northern horizon, until it reached the
    due west, where it rested, and formed for some time a luminous spot in
    the sky. About the same time a long low arch of light formed along the
    northern horizon, having a brighter patch at each extremity; and these being
    higher in the sky, the arch and turned-up ends were in shape like a Tartar
    bow. This bow was permanent; and later on a cloud of rose-coloured light
    formed in the east, looking like the reflection of a distant fire. From the
    bow also shot up curved streamers of silver light towards the zenith, which
    at one time threatened to form a corona. This, however, did not happen,
    and the Aurora gradually faded away, until, when the moon rose about 8,
    a silver tinge in the east alone remained. I should also mention that fleecy
    horizontal clouds of misty light floated in the north above the bow across the
    streamers.</p>

    <p>Mr. H. Taylor informed me he saw a similar Aurora some three weeks
    before, in which the bright horizontal light and short white streamers were
    the main characters. I am not sure that the horizontal light-clouds were not
    actual mist-clouds illuminated by reflection of the Aurora; not so, however,
    I think, the first-mentioned cloud, which had more the appearance of the
    <i>aura</i> in the large end of an illuminated Geissler tube.</p>

    <span class="sidenote">Spectrum of
    the Aurora
    described.</span>

    <p>I examined the Aurora with a Browning direct-vision spectroscope, and
    found Ångström’s line quite bright, and by the side of it three faint and misty
    bands towards the blue end of the spectrum upon a faintly illuminated
    ground. I could also see at times a bright line beyond the bands towards the
    violet. There was not light enough to take any measurements of position of
    the lines.</p>

    <p>I made a pencil sketch of this Aurora, at the time when the light-cloud
    had moved W. and the arc formed, and of the spectrum. These drawings are
    reproduced on Plate VI. figs. 1 and 1<i>a</i>.</p>

    <figure class="plate" id="plate6">
      <!-- <img src="assets/aurorae/images/plate6.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-10"><i>Mr. Herbert Ingall’s Aurora, July 18th, 1874.</i></h4>

    <span class="sidenote">Mr. Herbert
    Ingall’s
    Aurora, July
    18, 1874. Haze canopy
    formed. Bright
    bluish flames
    appeared. Beams and
    streamers
    appeared. Oscillatory
    motion of
    rays.</span>

    <p>An Aurora of July 18th, 1874, seen by Mr. Herbert Ingall at Champion
    Hill, S.E., was described by him as an extraordinary one. About 11 the
    sky was clear; at midnight the sky was covered by a sort of haze canopy,
    sometimes quite obscuring the stars, and then suddenly fading away. Mr.
    Ingall was shortly after remarking the sky in the S.E. and S. horizon as being
    more luminous than usual, when his attention was drawn to a growing
    brightness in the S.W., and a moment afterwards bright bluish flames
    “swept over the S.W. and W. horizon, as if before a high wind. They were
    not streamers, but bright blue flames.” They lasted about a minute and
    faded; but about two minutes afterwards a glowing luminosity appeared in
    the W.S.W., and broke into brilliant beams and streamers. The extreme
    rays made an angle of 90° with each other, the central ray reaching an
    altitude of 50°. The extreme divergence of the streamers (indicating their
    height above the earth’s surface), and their direction (from W.S.W. to E.N.E.)
    at right angles to the magnetic meridian, suggested to Mr. Ingall a disturbance
    of an abnormal character. The rays had an oscillatory motion for about
    fifteen seconds, and then disappeared, “as if a shutter had suddenly obscured
    the source of light.”</p>


    <span class="sidenote">Mr. Ingall’s
    remarks corroborated.</span>

    <p>Mr. Ingall’s remarks were corroborated by an observer in lat. 54° 46´ 6″·2 N.,
    long. 6h 12m 19s·75 W. The display, however, was more brilliant, and the
    intensity of light at midnight illuminated the whole district as with an electric
    light. The rays, too, bore tints differing from one another; the largest
    seemed to partake of the nature of the blue sky, while the smaller ones,
    running parallel with the horizon, were ever changing from blue to orange-red.</p>

    <span class="sidenote">Rev. C.
    Gape saw
    flashes or
    streaks of a
    pale blue
    colour.</span>

    <p>On June 25 (same year?), between 9 and 10 o’clock, the Rev. Chas. Gape
    saw at Rushall Vicarage, Scole, Norfolk, in the E.S.E., very frequent flashes
    or streaks of a pale blue colour darting from the earth towards the heavens
    like an Aurora. The day had been dull and close, with distant thunder.
    In the E.S.E. it was dark, but overhead and everywhere else it was clear
    and starry.</p>

    <h4 id="chap-3-11"><i>Mr. J. R. Capron’s White Aurora of September 11th, 1874.</i></h4>

    <span class="sidenote">Mr. J. R.
    Capron’s
    white
    Aurora of
    Sept. 11,
    1874.</span>

    <p>On September 11, 1874, we were at Kyle Akin, in the Isle of Skye. The
    day had been wet and stormy, but towards evening the wind fell and the
    sky became clear. About 10 <span class="smcapuc">P.M.</span> my attention was called to a beautiful
    Auroral display.</p>

    <span class="sidenote">Double arc
    of pure
    white light
    in the N.</span>

    <p>No crimson or rose tint was to be seen, but a long low-lying arc of the
    purest white light was formed in the north, and continued to shine with more
    or less brilliancy for some time. The arc appeared to be a double one, by the
    presence of a dark band running longitudinally through it.</p>

    <span class="sidenote">White
    streamers. Auroral bow
    believed to
    be near the
    earth.</span>

    <p>Occasional streamers of equally pure white light ran upwards from either
    end of the bow. The moon was only a day old, but the landscape was lighted
    up as if by the full moon; and the effect of Kyle Akin lighthouse, the
    numerous surrounding islands, and the still sea between was a true thing of
    beauty. The display itself formed a great contrast to the more brilliant
    but restless forms of Auroræ generally seen. I particularly noticed a somewhat
    misty and foggy look about the brilliant arc, giving it almost a solid
    appearance. The space of sky between the horizon and the lower edge of
    the arc was of a deep indigo colour, probably the effect of contrast. I had a
    strong impression that the bow was near to the earth, and was almost convinced
    that the eastern end and some fleecy clouds in which it was involved
    were between myself and the peaks of some distant mountains.</p>

    <p>I have not seen any other account of this Aurora, of which I was able at
    the time to obtain a sketch. This is reproduced on Plate VII. It was a
    lovely sight, and wonderfully unlike the cloud-accompanied and crimson
    Auroræ which I had seen in the South.</p>

    <p>It is noticed in Parry’s ‘Third Voyage’ that the lower edge of the auroral
    arch is generally well defined and unbroken, and the sky beneath it so exactly
    like a dark cloud (to him often of a brownish colour), that nothing could
    convince to the contrary, if the stars, shining through with undiminished
    lustre, did not discover the deception.</p>

    <span class="sidenote">No trace of
    brown colour
    in segment
    of sky below
    the arc.</span>

    <p>I saw no trace of brown colour. The segment below the arch resting on
    the horizon was of a deep indigo colour.</p>

    <figure class="plate" id="plate7">
      <!-- <img src="assets/aurorae/images/plate7.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-3-12"><i>Dr. Allnatt’s Aurora, June 9th, 1876.</i></h4>

    <span class="sidenote">Dr. Allnatt’s
    Aurora,
    June 9, 1876. Band of
    auroral light
    appeared. Streaks of
    cirro-stratus
    divided the
    Aurora. Want of
    electric
    manifestations
    attributed
    to
    absence of
    sun-spots.</span>

    <p>Dr. Allnatt, writing to the ‘Times’ from Abergele, North Wales, near the
    coast of the Irish Channel, reported an Aurora on the night of the 9th
    June, 1876. After a cool and gusty day, with a strong N.E. wind and a
    disturbed sea, there appeared at 11 <span class="smcapuc">P.M.</span> in the N. horizon a broad band of
    vivid auroral light, homogeneous, motionless, and without streamers. About
    midnight a long attenuated streak of black cirro-stratus stretched parallel
    with the horizon, and divided the Aurora into nearly symmetrical sections.
    On the preceding day the sky was covered with dark masses of electric cloud
    of weird and fantastic forms. The season had been singularly unproductive
    of high electric manifestations, which Dr. Allnatt thought might be attributable
    to the comparative absence of spots on the solar disk. [It may here
    be noted how conspicuous the years 1877 and 1878 have been for absence of
    Sun-spots and of Auroræ.]</p>

    <figure class="plate" id="plate8">
      <!-- <img src="assets/aurorae/images/plate8.jpg" /> -->
      <figcaption></figcaption>
    </figure>


    <h4 id="chap-3-13"><i>Herr Carl Bock’s Lapland Aurora, 3rd October, 1877.</i></h4>

    <span class="sidenote">Herr Carl
    Bock’s Lapland
    Aurora,
    3rd Oct.
    1877. Lapland
    Auroræ
    generally of
    the yellow
    type.</span>

    <p>In January 1878 I had the pleasure to meet, at the Westminster Aquarium,
    Herr Carl Bock, the Norwegian naturalist, who accompanied four Laplanders,
    two men and two women, with sledges, tents, &amp;c., on their visit to this
    country. The Laplanders (as mentioned elsewhere) did not confirm the
    accounts of noises said to have been observed by Greenlanders and others
    during the Aurora. Carl Bock mentioned to me that the displays he saw in
    Lapland were most brilliant, but generally of the yellow type (the Laplanders
    called the Aurora “yellow lights”). He saw only one red Aurora.
    He kindly lent me a picture (probably in its way unique), an oil-painting of
    an Aurora Borealis, entirely sketched by the light of the Aurora itself.</p>

    <span class="sidenote">A picture
    painted by
    light of the
    Aurora. Movement
    of the rays. Inner edge
    of arc
    fringed with
    rays.</span>

    <p>The painting is remarkable for the tender green of the sky, an effect
    probably due to a mixture of the ordinary sky colour with the yellow light of
    the Aurora. This picture was taken at Porsanger Fjord, in lat. 71° 50´, on
    3rd October, 1877. It lasted from 9 <span class="smcapuc">P.M.</span> till about 11 <span class="smcapuc">P.M.</span> The rays kept
    continually moving, and certain of them seemed in perspective and behind
    the others. It will be noticed that the <i>inner</i> edge of the arc is fringed with
    rays, contrary to the sharp and definite margin which is usually presented.
    Probably two Auroræ or auroral forms were seen—a quiescent arc in front,
    and a set of moving streamers beyond. Two larger and brighter patches of
    light are seen at each extremity of the arc, as in the case of the Aurora seen
    by me at Guildown, February 4th, 1874, which, indeed, the display much
    resembles. A reduced facsimile of Herr Bock’s excellent picture is given on
    Plate VIII.</p>

    <span class="sidenote">Aurora of
    longitudinal
    rays.</span>

    <p>Herr Bock also acquainted me that on the following day he saw an Aurora
    in which the lines of light, instead of being vertical, were longitudinal, and
    were continually swept along in several currents. They were not so strong
    as in the former case.</p>

    <h4 id="chap-3-14"><i>Rev. T. W. Webb’s Aurora.</i></h4>

    <span class="sidenote">Rev. T. W.
    Webb’s
    Aurora. Arc resolved
    into sets of
    streamers
    moving in
    opposite
    directions.</span>

    <p>The Rev. T. W. Webb has described to me in a letter an Aurora very like
    that seen by Carl Bock in Lapland, and apparently the prevailing type in
    those regions. An arc similar to that figured by Carl Bock appeared in the
    N.W., and seemed to resolve itself into two sets of streamers moving in
    opposite directions (or the one set might be fixed and the other moving), like
    the edges of two great revolving toothed wheels. This lasted but for a few
    seconds; but during that interval the tints were varied and brilliant, including
    blue and green.</p>

    <h4 id="chap-3-15"><i>The English Arctic Expedition 1875-76, under Capt. Sir George Nares.</i></h4>

    <span class="sidenote">English
    Arctic
    Expedition,
    1875-76. Instructions
    for use of
    officers. Appendix B. Capt. Sir
    G. Nares’s
    report. True Auroræ
    seldom observed,
    and
    displays
    faint. Citron-line
    observed on
    only two
    displays. Appendix C.</span>

    <p>In anticipation of the starting of this Expedition, some instructions for the
    use of the officers in connexion with the hoped for display of brilliant Auroræ
    were prepared:—as to general features of the Auroræ, by Professor Stokes; as
    to Polarization, by Dr. William Spottiswoode; and as to Spectrum work, by
    Mr. Norman Lockyer and myself. As these instructions were somewhat
    elaborate, and will apply to all Auroral displays, I have supplied a copy
    of them in Appendix B. They were unfortunately not brought into
    requisition, for want of the Auroræ themselves. Capt. Sir George Nares
    has reported to the Admiralty, under date 5th December, 1877, as follows:—“Although
    the auroral glow was observed on several occasions between
    25 October, 1875, and 26 February, 1876, true Auroræ were seldom observed;
    and the displays were so faint, and lasted so short a time, and the spectrum
    observations led to such poor results, that no special report has been considered
    necessary. Although the citron-line was observed occasionally, on
    only two displays of the Aurora was it well defined, and then for so short a
    time that no measure could be obtained.” (For Sir George Nares’s further
    Report see Appendix C, containing extracts from blue-book, ‘Results derived
    from the Arctic Expedition, 1875-6.’)</p>

    <h4 id="chap-3-16"><i>Aurora Australis.</i></h4>

    <span class="sidenote">Aurora
    Australis. Mr. Forster’s
    description. Long
    columns of
    white light
    spreading
    over the
    whole sky.</span>

    <p>In an article on Auroræ in high Southern latitudes (Phil. Trans. No. 461,
    and vol. liv. No. 53), we find that Mr. Forster, who as naturalist accompanied
    Capt. Cook on his second voyage round the world, says:—“On
    February 17th, 1773, in south latitude 58°, a beautiful phenomenon was
    observed during the preceding night, which appeared again this, and several
    following nights. It consisted of long columns of a clear white light
    shooting up from the horizon to the eastward almost to the zenith, and
    gradually spreading over the whole southern part of the sky. These columns
    were sometimes bent sideways at their upper extremities; and though in most
    respects similar to the northern lights of our hemisphere, yet differed from
    them in being always of a whitish colour, whereas ours assume various tints,
    especially those of a fiery and purple hue. The sky was generally clear when
    they appeared, and the air sharp and cold, the thermometer standing at the
    freezing point.” This account agrees very closely in particulars with Capt.
    Maclear’s notice of Aurora Australis [after referred to], and especially in the
    marked absence of red Auroræ.</p>

    <p>The height of the barometer does not appear to be mentioned, the temperature
    being apparently much the same as in the more recent cases.</p>

    <span class="sidenote">Capt.
    Maclear’s
    Aurora
    Australis,
    3rd March,
    1874. Light of
    pale yellow
    tint only.</span>

    <p>In a letter dated from H.M.S. ‘Challenger,’ North Atlantic, April 10th,
    1876, Capt. Maclear was good enough to communicate to me some particulars
    of an Aurora Australis seen 3rd March 1874, in lat. 54° S., long. 108° E.
    The letter is mainly descriptive of the spectrum (which will be described in
    connexion with the general question of the spectrum of the Aurora). It
    states that the red line was looked for in vain, and that the light appeared
    of a <i>pale yellow</i>, and had none of the rosy tint seen in the northern
    displays.</p>

    <span class="sidenote">Capt.
    Maclear’s
    Auroræ
    described in
    ‘Nature.’</span>

    <p>Capt. Maclear has since contributed to ‘Nature,’ of 1st November 1877, a
    description of four Auroræ seen from the ‘Challenger’ in high southern
    latitudes (including the one communicated to me). He speaks of the opportunity
    of observing as not frequent, either from the rarity of the phenomena,
    or because the dense masses of cloud prevalent in those regions prevented
    their being seen except when exceptionally bright. There were four appearances
    described:—</p>

    <span class="sidenote">Feb. 9, 1874.</span>

    <p>(1.) At 1.30 on the morning of February 9th, 1874, preceded by a watery
    sunset, lat. 57° S. and long. 75° E., bar. 29·0 in., ther. 35°; brilliant streaks
    to the westward. Day broke afterwards with high cirrus clouds and clear
    horizon.</p>

    <span class="sidenote">Feb. 21,
    1874.</span>

    <p>(2.) At 9.30 <span class="smcapuc">P.M.</span>, February 21, 1874, lat. 64° S., long. 89° E., bar. 28·8 in.,
    ther. 31°; one bright curved streamer. The Aurora preceded a fine morning
    with cumulo-stratus clouds, extending from Jupiter (which appeared to be
    near the focus) through Orion and almost as far beyond. Under this a black
    cloud, with stars visible through it. Real cumuli hid great part of the
    remainder of the sky, but there were two vertical flashing rays which moved
    slowly to the right (west). Generally the Aurora was still bright.</p>

    <span class="sidenote">March 3,
    1874. Auroral line
    found in
    light to
    southward.</span>

    <p>(3.) At midnight, March 3rd, 1874, lat. 53° 30´ S., long. 109° E., bar. 29·1,
    ther. 36°, after some days’ stormy weather, a brilliant sunset, followed by a
    fine morning. Soon after 8 <span class="smcapuc">P.M.</span> the sky began to clear and the moon shone
    out. Noticing the light to the southward to be particularly bright, Capt.
    Maclear applied the spectroscope, and found the distinguishing auroral line.</p>

    <span class="sidenote">Brilliant
    white clouds
    seen.</span>

    <p>About midnight the sky was almost clear, but south were two or three
    brilliant light clouds, colour very white-yellow, shape cumulo-stratus. From
    about west to near south extended a long feathery light of the same colour,
    parallel with the horizon, and between south and west there appeared occasionally
    brilliant small clouds. The upper edges seemed hairy, and gave one
    the idea of a bright light behind a cloud. The forms changed, but no particular
    order was noticed.</p>

    <p>(Here follows a description of the spectrum, and the mode in which a
    delineation by the lines was obtained.)</p>

    <span class="sidenote">March 6,
    1874. Capt.
    Maclear
    suggests
    whether a
    low barometer
    has
    to do with
    the absence
    of red.</span>

    <p>(4.) At 8 <span class="smcapuc">P.M.</span>, March 6th, 1874. This was a slight Aurora, seen to the
    southward; after this the clouds changed to high cirrus. Capt. Maclear
    suggests whether a low barometer has any thing to do with the absence of red
    in the spectrum, the normal state of the barometer being an inch lower in
    those regions than in more temperate latitudes.</p>

    <span class="sidenote">Barometer
    falls after
    the Aurora,
    and strong
    gale from
    the S. or S.W.
    follows.</span>

    <p>Edin. Encyc. vol. iii. article “Aurora.” Dr. Kirwan observed that the
    barometer commonly falls after the Aurora. Mr. Winn, in the seventy-third
    volume of the Phil. Trans., makes the same remark, and says that in twenty-three
    instances, without fail, a strong gale from the south or south-west
    followed the appearance of an Aurora. If the Aurora were bright, the gale
    came on within twenty-four hours, but was of no long continuance; if the
    light was faint and dull, the gale was less violent, longer in coming, and longer
    in duration.</p>

    <span class="sidenote">Pale yellow
    glow rare in
    the Aurora
    Borealis.</span>

    <p>The pale yellow-coloured glow referred to by Capt. Maclear is, in my experience,
    rare in the Aurora Borealis. It is probably the “æqualiter et sine
    eruptionibus aut radiis <i>fulvi</i>,” described by Seneca (<i>antè</i>, p. 1), and may probably
    belong to more southern climes.</p>

    <span class="sidenote">Spectrum
    of Auroræ
    Australes
    extends
    more into
    the violet.</span>

    <p>We shall see too, by-and-by, that these Auroræ Australes as to spectrum
    extend more into the violet than the Aurora Borealis. The yellow, as complementary
    to violet, is likely thus to make (in the absence of the red) its
    appearance.</p>

    <p>It is, however, somewhat singular that Carl Bock found almost exclusively
    yellow Auroræ in Lapland.</p>

    <p>In Proctor’s ‘Borderland of Science,’ article “The Antarctic Regions,” we
    find quoted a passage from a letter by Capt. Howes, of the ‘Southern Cross,’
    in which a graphic description is given of a Southern Aurora:—</p>

    <span class="sidenote">Capt.
    Howes’s description
    of
    a Southern
    Aurora.</span>

    <p>“At about half-past one on the 2nd of last September the rare phenomenon
    of the Aurora Australis manifested itself in a most magnificent manner.
    Our ship was off Cape Horn, in a violent gale, plunging furiously into a
    heavy sea, flooding her decks, and sometimes burying her whole bows beneath
    the waves. The heavens were as black as death, not a star was to be seen,
    when the brilliant spectacle first appeared.</p>


    <span class="sidenote">Balls of
    electric fire
    resting on
    mast-heads
    &amp;c.</span>

    <p>“I cannot describe the awful grandeur of the scene; the heavens gradually
    changed from murky blackness till they became like vivid fire, reflecting a
    lurid glowing brilliancy over every thing. The ocean appeared like a sea of
    vermilion lashed into fury by the storm, the waves dashing furiously over our
    side, ever and anon rushed to leeward in crimson torrents. Our whole ship—sails,
    spars, and all—seemed to partake of the same ruddy hues. They were
    as if lighted up by some terrible conflagration. Taking all together—the
    howling, shrieking storm, the noble ship plunging fearlessly beneath the
    crimson-crested ways, the furious squalls of hail, snow, and sleet, drifting
    over the vessel, and falling to leeward in ruddy showers, the mysterious
    balls of electric fire resting on our mast-heads, yard-arms, &amp;c., and, above all,
    the awful sublimity of the heavens, through which coruscations of auroral
    light would shoot in spiral streaks, and with meteoric brilliancy,—there was
    presented a scene of grandeur surpassing the wildest dreams of fancy.”</p>

    <p>The foregoing picture presents a singular contrast to the yellow-white
    Auroræ described as seen in high southern latitudes by Capt. Maclear, and is
    interesting as a southern Aurora of a red or ruddy tint. Looking, however,
    at the extreme rarity of red Auroræ in those latitudes, and the description of
    “mysterious balls of electric fire resting on our mast-heads, yard-arms, &amp;c.” (a
    phenomenon not often noticed in connexion with the Aurora), it suggests
    itself that the case in question may have been an instance not of a true
    Aurora, but of an electric display, with conditions approaching those experienced
    by travellers who have found themselves in mountainous districts
    surrounded by storm-clouds charged with electricity&nbsp;<span class="footnote">Some curious instances have been recently (January 1879) given in the ‘Times’ of such electric phenomena, comprising, amongst others, gas lighted by the finger in Canada, points of flame seen on the ironwork of Teignmouth Bridge, and similar points seen on the alpenstocks and axes of a party making a mountain ascent in Switzerland.</span>.</p>

    <h4 id="chap-3-17"><i>Prof. Piazzi Smyth’s Typical Auroræ.</i></h4>

    <span class="sidenote">Prof. Piazzi
    Smyth’s
    typical
    Auroræ.</span>

    <p>Prof. Piazzi Smyth was kind enough lately to send me the fourteenth
    volume of the ‘Astronomical Observations made at the Royal Observatory,
    Edinburgh, during the years 1870-1877.’ This volume, amongst its other
    interesting matter, affords some valuable information on the subject of the
    Aurora Borealis. The Aurora plates are five in number, three comprising
    some well-executed chromo-lithographs of typical Auroræ, from sketches
    made by Prof. Smyth, the other two plates being of the Aurora spectrum.
    The Auroræ delineated are thus described:—</p>


    <span class="sidenote">Aug. 6, 1871,
    quiescent
    arc. August 21,
    1871,
    active arc.</span>

    <p>Plate 5. (August 6, 1871.) An example of a mild quiescent kind of
    auroral arc, with dark cavernous substratum. (August 21, 1871.) An example
    of a bright large active arc darting out rays.</p>

    <span class="sidenote">Sept. 7, 1871,
    arc streamers
    and
    clouds. May 8, 1871,
    double arc
    (longitudinal).</span>

    <p>Plate 6. (September 7, 1871.) An auroral arc, with streamers and dark
    clouds, and maintaining a bright appearance though in proximity to the
    moon. (May 8, 1871.) A double-arched auroral arc (the arches are longitudinally
    arranged).</p>

    <span class="sidenote">April 28,
    1871, multiple
    arc. Oct. 25,
    1870,
    coloured
    Aurora.</span>

    <p>Plate 7. (April 28, 1871.) A multiple-arched arc of Aurora with moonlight.
    (October 25, 1870.) A case of grandest coloured Auroræ, or Aurora
    superb and almost universal.</p>

    <p>All the foregoing drawings are very vivid and striking, and form a most
    interesting set of typical forms of Auroræ.</p>

    <p>According to my own experience, the Aurora with arches arranged longitudinally,
    thus, <img src="assets/aurorae/images/i_030.jpg" width="60" height="18" alt="Symbols" />, is the rarest of all the forms. I have not met
    with it myself, nor do I recollect an illustration of one other than Prof.
    Smyth’s.</p>

  </section>

  <!-- Chapitre 4_________________________________________________________-->
  <section class="chapter" id="chapter-4">
    <h3 class="titlechapter" id="chap-4">Phenomena simuling auroræ</h3>
    <p class="shorter">Phenomena simuling auroræ</p>

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    <div id="chap-4-flow-fr">
      <h4 id="chap-4-1"><i>Auroric Lights (Kinahan).</i></h4>


      <p>Mr. G. Henry Kinahan writes to ‘Nature,’ from Ovoca, under date
      January 27th, 1877, and speaks of two distinct kinds of light so classed—one
      brilliant and transparent, of a white yellowish-blue or yellowish-red colour,
      while the other is semi-opaque and of a bloody red colour, the latter being
      considered in Ireland a forerunner of bad weather. The first kind generally
      appears as intermittent pencils of light that suddenly appear and disappear.</p>

      <div class="sidenote">Frequently
      not stationary,
      but
      jumping
      about.</div>

      <p>Usually they proceed or radiate from some point near the north of the
      horizon; but Mr. Kinahan has frequently seen them break from a point in
      the heavens, not stationary, but jumping about within certain limits. Sometimes
      these lights occur as suddenly flashing clouds of light of a white colour,
      but at other times of blue and reddish yellow.</p>

      <div class="sidenote">In daylight
      like sun-rays. Red light
      appears in
      clouds floating
      upwards
      or diffused.</div>


            <div class="sidenote">Frequently
            not stationary,
            but
            jumping
            about.</div>

            <p>Usually they proceed or radiate from some point near the north of the
            horizon; but Mr. Kinahan has frequently seen them break from a point in
            the heavens, not stationary, but jumping about within certain limits. Sometimes
            these lights occur as suddenly flashing clouds of light of a white colour,
            but at other times of blue and reddish yellow.</p>

            <div class="sidenote">In daylight
            like sun-rays. Red light
            appears in
            clouds floating
            upwards
            or diffused.</div>

            <p>If this class of lights is watched into daylight, they appear somewhat like
            faint rays of a rising sun. One morning, while travelling in West Galway in
            the twilight, they were very brilliant, and quite frightened Mr. Kinahan’s
            car-driver, who thought the sun was going to rise in the north instead of the
            east. The second, or bloody red light, usually occurs in clouds floating in
            one direction up into the heavens, but often diffused over a portion of the sky.
            Mr. Kinahan has never seen them coming from the east, and on only a few
            occasions from the south, but generally from the west, north-west, or north.</p>

            <div class="sidenote">Red light
            appears as
            dirty misty
            clouds in
            daylight,
            or as a mist
            or misty
            rays.</div>

            <p>If both kinds of light appear at the same time, the second while passing
            over the first dims it. If the second class is watched into daylight, they
            appear as dirty misty clouds that suddenly form and disappear without the
            spectator being able to say where they come from or where they go to, or as
            a hazy mist over a portion of the sky, that suddenly appears and disappears, or
            as misty rays proceeding from a point in the horizon. Generally, when
            these clouds occur, there is a bank of black clouds to the westward.</p>

            <div class="sidenote">Season since
            October
            1876 prolific
            in auroric
            light.</div>

            <p>Mr. Kinahan then speaks of the season as having been prolific in auroric
            light, as there had been few nights since the 1st October then last (1876) in
            which they did not appear. On many occasions they were late in the night,
            being very common and brilliant during the dark days of December, a few
            hours before dawn (about 5 o’clock). Each time there was a fine day they
            appeared also, and the weather broke again.</p>

            <div class="sidenote">Mr. J. Allan
            Broun questions
            nature
            of these
            lights, as
            Aurora is
            seldom seen
            at 5 <span class="smcapuc">A.M.</span> in
            this country. On 77 occasions
            seen
            only twice
            so early. Season was
            of marked
            infrequency
            elsewhere.</div>

            <p>Mr. Jno. Allan Broun refers to this graphic account of Mr. Kinahan’s,
            and concludes there must have been some mistake as to the nature of these
            “auroric lights,” as the Aurora Borealis is very rarely seen at 5 <span class="smcapuc">A.M.</span> in this
            country. In the years 1844 and 1845, during which the Aurora was sought for
            at Makerstown every hour of the night, it was observed in 77 nights on an
            average of nearly three hours each night; but it was seen only twice so early,
            and that with a bright or brilliant Aurora, which remained during five hours
            on the first occasion, and from 6 <span class="smcapuc">P.M.</span> to 6 <span class="smcapuc">A.M.</span> on the second. Parts of the
            phenomenon seen by Mr. Kinahan, Mr. Broun also could not say he had ever
            seen; and if Mr. Kinahan’s observations could have been confirmed it would
            have been most important, especially as made so frequently at the epoch of
            minimum. The description is in many respects a sufficiently recognizable
            one of auroral discharges; but the frequent appearance in early morning is
            certainly unusual, and few if any Auroræ seem to have been recorded as
            appearing elsewhere in Great Britain during the time which Mr. Kinahan refers
            to as so prolific (see, however, Dr. Allnatt’s, <i>antè</i>, p. 24). In fact, the season
            in question was one of marked infrequency (see English Arctic Expedition
            Report, <i>antè</i>, p. 26). Mr. Buchan furnished Mr. Broun with a note of Auroræ
            seen in the stations of the Scottish Meteorological Society during the year
            1876, and they were 42 in number, 26 in the first half, and 16 in the second
            half of the year. The greater part were seen in the most northerly stations,
            including the Orkney, Shetland, and Faroe Islands, and only 9 south of
            the Forth.</p>

            <h4 id="chap-4-2"><i>Luminous Arch.</i></h4>

            <div class="sidenote">Luminous
            arch, Sept.
            11, 1814. Height
            above horizon
            6 to 9
            miles.</div>

            <p>In the ‘Annals of Philosophy,’ vol. iv. p. 362, there is a minute description
            of a luminous arch which appeared in the sky on the night of Sunday,
            September 11th, 1814, and was seen in the west of England opposite the
            Irish Sea, the west part of the south of Scotland, and part of the west of
            Ireland. It was described as a part of either a body of dense greyish-white
            light, or a mass of luminous matter in the shape of an arch. Its height above
            the horizontal line was estimated at not more than 9 nor less than 6 miles.</p>

            <div class="sidenote">It moved
            southward,
            and was
            assumed to
            differ from
            the Aurora.</div>

            <p>Its direction when first seen was N. 80° E., and S. 80° W. It moved to the
            southward. It was assumed to differ from the Aurora Borealis in wanting
            coruscations, and in its having a much paler light.</p>

    </div>






    <div id="chap-4-flow-eng">


          <p>Mr. G. Henry Kinahan writes to ‘Nature,’ from Ovoca, under date
          January 27th, 1877, and speaks of two distinct kinds of light so classed—one
          brilliant and transparent, of a white yellowish-blue or yellowish-red colour,
          while the other is semi-opaque and of a bloody red colour, the latter being
          considered in Ireland a forerunner of bad weather. The first kind generally
          appears as intermittent pencils of light that suddenly appear and disappear.</p>

          <div class="sidenote">Frequently
          not stationary,
          but
          jumping
          about.</div>

          <p>Usually they proceed or radiate from some point near the north of the
          horizon; but Mr. Kinahan has frequently seen them break from a point in
          the heavens, not stationary, but jumping about within certain limits. Sometimes
          these lights occur as suddenly flashing clouds of light of a white colour,
          but at other times of blue and reddish yellow.</p>

          <div class="sidenote">In daylight
          like sun-rays. Red light
          appears in
          clouds floating
          upwards
          or diffused.</div>

          <p>If this class of lights is watched into daylight, they appear somewhat like
          faint rays of a rising sun. One morning, while travelling in West Galway in
          the twilight, they were very brilliant, and quite frightened Mr. Kinahan’s
          car-driver, who thought the sun was going to rise in the north instead of the
          east. The second, or bloody red light, usually occurs in clouds floating in
          one direction up into the heavens, but often diffused over a portion of the sky.
          Mr. Kinahan has never seen them coming from the east, and on only a few
          occasions from the south, but generally from the west, north-west, or north.</p>

          <div class="sidenote">Red light
          appears as
          dirty misty
          clouds in
          daylight,
          or as a mist
          or misty
          rays.</div>

          <p>If both kinds of light appear at the same time, the second while passing
          over the first dims it. If the second class is watched into daylight, they
          appear as dirty misty clouds that suddenly form and disappear without the
          spectator being able to say where they come from or where they go to, or as
          a hazy mist over a portion of the sky, that suddenly appears and disappears, or
          as misty rays proceeding from a point in the horizon. Generally, when
          these clouds occur, there is a bank of black clouds to the westward.</p>

          <div class="sidenote">Season since
          October
          1876 prolific
          in auroric
          light.</div>

          <p>Mr. Kinahan then speaks of the season as having been prolific in auroric
          light, as there had been few nights since the 1st October then last (1876) in
          which they did not appear. On many occasions they were late in the night,
          being very common and brilliant during the dark days of December, a few
          hours before dawn (about 5 o’clock). Each time there was a fine day they
          appeared also, and the weather broke again.</p>

          <div class="sidenote">Mr. J. Allan
          Broun questions
          nature
          of these
          lights, as
          Aurora is
          seldom seen
          at 5 <span class="smcapuc">A.M.</span> in
          this country. On 77 occasions
          seen
          only twice
          so early. Season was
          of marked
          infrequency
          elsewhere.</div>

          <p>Mr. Jno. Allan Broun refers to this graphic account of Mr. Kinahan’s,
          and concludes there must have been some mistake as to the nature of these
          “auroric lights,” as the Aurora Borealis is very rarely seen at 5 <span class="smcapuc">A.M.</span> in this
          country. In the years 1844 and 1845, during which the Aurora was sought for
          at Makerstown every hour of the night, it was observed in 77 nights on an
          average of nearly three hours each night; but it was seen only twice so early,
          and that with a bright or brilliant Aurora, which remained during five hours
          on the first occasion, and from 6 <span class="smcapuc">P.M.</span> to 6 <span class="smcapuc">A.M.</span> on the second. Parts of the
          phenomenon seen by Mr. Kinahan, Mr. Broun also could not say he had ever
          seen; and if Mr. Kinahan’s observations could have been confirmed it would
          have been most important, especially as made so frequently at the epoch of
          minimum. The description is in many respects a sufficiently recognizable
          one of auroral discharges; but the frequent appearance in early morning is
          certainly unusual, and few if any Auroræ seem to have been recorded as
          appearing elsewhere in Great Britain during the time which Mr. Kinahan refers
          to as so prolific (see, however, Dr. Allnatt’s, <i>antè</i>, p. 24). In fact, the season
          in question was one of marked infrequency (see English Arctic Expedition
          Report, <i>antè</i>, p. 26). Mr. Buchan furnished Mr. Broun with a note of Auroræ
          seen in the stations of the Scottish Meteorological Society during the year
          1876, and they were 42 in number, 26 in the first half, and 16 in the second
          half of the year. The greater part were seen in the most northerly stations,
          including the Orkney, Shetland, and Faroe Islands, and only 9 south of
          the Forth.</p>


          <div class="sidenote">Luminous
          arch, Sept.
          11, 1814. Height
          above horizon
          6 to 9
          miles.</div>

          <p>In the ‘Annals of Philosophy,’ vol. iv. p. 362, there is a minute description
          of a luminous arch which appeared in the sky on the night of Sunday,
          September 11th, 1814, and was seen in the west of England opposite the
          Irish Sea, the west part of the south of Scotland, and part of the west of
          Ireland. It was described as a part of either a body of dense greyish-white
          light, or a mass of luminous matter in the shape of an arch. Its height above
          the horizontal line was estimated at not more than 9 nor less than 6 miles.</p>

          <div class="sidenote">It moved
          southward,
          and was
          assumed to
          differ from
          the Aurora.</div>

          <p>Its direction when first seen was N. 80° E., and S. 80° W. It moved to the
          southward. It was assumed to differ from the Aurora Borealis in wanting
          coruscations, and in its having a much paler light.</p>
    </div>

  </section>

  <!-- Chapitre 5_________________________________________________________-->
  <section class="chapter" id="chapter-5">
    <h3 class="titlechapter" id="chap-5">Some qualities of the aurora</h3>
    <p class="shorter">Some qualities of the aurora</p>



    <h4 id="chap-5-1"><i>Noises attending Auroræ.</i></h4>

    <span class="sidenote">Sir John Franklin negatives them.</span>



    <p>In the Edinb. Encyc., Gmelin is stated, in continuation of his description of
    an Arctic Aurora, to add:—“For however fine the illumination may be, it is
    attended, as I have heard from the relation of many persons, with such a
    hissing, cracking, and rushing noise through the air, as if the largest fireworks
    were playing off.” To describe what they then heard, the natives are
    said to use the expression, “Spolochi chodjat”—that is, The raging host is passing.
    The hunter’s dogs, too, are also described as so much frightened when
    the Auroræ overtake the hunters, that they will not move, but lie obstinately
    on the ground till the noise has passed. This account of noises seems to be
    confirmed by other testimony. They are stated to have been heard at Hudson’s
    Bay and in Sweden; and Musschenbroek mentions that the Greenland
    whale-fishers assured him they had frequently heard the noise of the Aurora
    Borealis, but adds that “no person in Holland had ever experienced this
    phenomenon.” Mr. Cavallo declares he “has repeatedly heard a crackling
    sound proceeding from the Aurora Borealis” (Elements of Nat. or Exper.
    Phil. vol. iii. p. 449). Mr. Nairne mentions that in Northampton, when the
    northern lights were very bright, he is confident he perceived a hissing or
    whizzing sound. Mr. Belknap of Dover, New Hampshire, North America,
    testifies to a similar fact (American Trans. vol. ii. p. 196).</p>

    <div class="sidenote">Noises attending
    Auroræ. Gmelin affirms
    them. Other testimony
    to
    them. Musschenbroek. Cavallo. Nairne. Belknap.</div>


    <p>Sir John Franklin mentions, in his ‘Journey to the Shores of the Polar
    Sea’:—“Nor could we distinguish its (the Aurora’s) rustling noise, of which,
    however, such strong testimony has been given to us that no doubt can remain
    of the fact.”</p>

    <div class="sidenote">March 11th.
    Hissing
    noise heard
    during Aurora’s
    passage. Explained
    to arise from
    the snow.</div>

    <p>In detail, he mentions he never heard any sound that could be unequivocally
    considered as originating in the Aurora, although he had had an opportunity
    of observing that phenomenon for upwards of 200 nights (the Aurora
    was registered at Bear Lake 343 times without any sound being heard to
    attend its motions); but the uniform testimony of the natives and all the
    older residents in the country induced him to believe that its motions were
    sometimes audible. On the 11th March, at 10 <span class="smcapuc">P.M.</span>, a body of Aurora rose
    N.N.W.; and after a mass had passed E. by S., the remainder broke away
    in portions, which crossed about 40° of the sky with great rapidity. A hissing
    noise, like that of a bullet passing through the air, was heard, which seemed
    to proceed from the Aurora; but Mr. Wentzel assured the party the noise
    was occasioned by severe cold succeeding mild weather, and acting upon the
    surface of the snow previously melted in the sun’s rays. A similar noise was
    heard the next morning.</p>

    <div class="sidenote">Capt. Sabine
    also negatives
    noise.</div>

    <p>In Parry’s first voyage, Captain Sabine describes an Aurora seen at Melville
    Island, and adds that the Aurora had the appearance of being <i>very near</i> the
    party, but <i>no sound could be heard</i>.</p>

    <div class="sidenote">Article
    “Aurora
    Polaris,”
    Encyc. Brit.,
    suggests
    noises as
    not improbable.</div>

    <p>In the article “Aurora Polaris,” Encyc. Brit, edition ix., the writer admits
    the evidence of scientific Arctic voyagers having listened in vain for such
    noises; but, referring to the statements of Greenlanders and others on the
    subject, concludes there is no <i>à priori</i> improbability of such sounds being
    occasionally heard, since a somewhat similar sound accompanies the brush-discharge
    of the electric machine.</p>

    <div class="sidenote">Payer negatives
    and
    discredits
    noises.</div>

    <p>Payer, of the Austrian Polar Expedition (1872-1874), states that the
    Aurora was never accompanied by noise, and discredits the alleged accounts
    of noises in the Shetlands and Siberia.</p>

    <span class="sidenote">As also Weyprecht.</span>

    <p>Lieut. Weyprecht, of the same expedition, says (<i>antè</i>, p. 14):—“Involuntarily
    we listen; such a spectacle must, we think, be accompanied with sound, but
    unbroken silence prevails, not the least sound strikes on the ear.”</p>

    <div class="sidenote">Herr Carl
    Bock negatives
    noises
    in the case
    of Lapland
    Auroræ.</div>

    <p>Herr Carl Bock, who accompanied the Laplanders visiting this country (at the
    Westminster Aquarium) in 1877-78, and who witnessed many brilliant auroral
    displays in Lapland, assured me he could trace no noise, except on one occasion,
    when he heard a sort of rustling, which he attributed to the wind. The
    Laplanders themselves did not associate any special noise with the Aurora.</p>

    <div class="sidenote">Auroral
    noises in
    telephone. Ringing
    sound in
    vacuum-tube
    under influence
    of
    magnet.</div>

    <p>It has been recently stated, in an article on the Telephone in ‘Nature,’ that
    Professor Peirce “has observed the most curious sounds produced from a
    telephone in connexion with a telegraph wire during the Aurora Borealis;”
    but no further details are given. In experimenting with a silicic fluoride
    vacuum-tube between the poles of an electro-magnet, I found, on the magnet
    being excited, that the capillary stream of blue light was decreased in volume
    and brightness, and at the same time from within the tube a peculiar whistling
    or slightly metallic ringing sound was heard.</p>

    <div class="sidenote">Adverse
    conclusion
    as to noises
    accompanying
    Aurora.</div>

    <p>I certainly have never met with an instance of noise accompanying an
    Aurora and traced to it. On the whole the balance of evidence seems quite
    adverse to any proof of noises proper ordinarily accompanying an Aurora.</p>

    <p></p>

    <h4 id="chap-5-2"><i>Colours of the Aurora.</i></h4>

    <div class="sidenote">Colours of
    the Aurora. Sir John
    Franklin’s
    views. Other observers
    have
    described all
    colours of
    spectrum. Violet rare.
    Crimson
    indicates
    coming
    Aurora.</div>

    <p>Sir John Franklin considered the colours in the Polar Aurora did not
    depend on the presence of any luminary, but were generated by the motion
    of the beams, and then only when that motion was rapid and the light brilliant.
    The lower extremities, he says, quivered with a fiery red colour, and
    the upper with orange. He also saw violet in the former. Other observers
    have, in their various descriptions of Auroræ, mentioned the colours of the
    rays or beams as red, crimson, green, yellow, &amp;c.; in fact, comprising the
    range of the spectrum. Violet seems less frequently mentioned. The red
    or crimson colour is frequently the first indication of the coming Aurora, and
    is usually seen on or near the horizon. The colours have frequently been
    observed to shift or change.</p>

    <div class="sidenote">Prof. Piazzi
    Smyth describes
    colours
    of Aurora
    of Feb.
    4, 1872, as
    seen at
    Edinburgh.</div>

    <p>Prof. Piazzi Smyth, in a letter to ‘Nature,’ describing the Aurora of
    February 4th, 1872, as seen at Edinburgh, says that when the maximum development
    was reached all the heavens were more or less covered with pink
    ascending streamers, except towards the N., which was dark and grey—first by
    means of a long low arch of blackness, transparent to large stars, and then by
    the streamers which shot up from this arch, which were green and grey only
    for several degrees of their height, and only became pink as they neared the
    zenith. The red streamers varied from orange to rose-pink, red rose, and
    damask rose.</p>

    <p>The Professor pointed out that the spectroscope knew no variety of reds
    giving one red line only, and attributed this to the mixing up of rays and
    streamers of blackness out of the long low arch. When the Aurora faded
    away a true starlight-night sky appeared; so that evidently the dark arch and
    streamers were as much part of the Aurora as the green and red lights.</p>

    <div class="sidenote">Dr. Allnatt
    at Frant
    describes
    vivid colours
    of same
    Aurora.</div>

    <p>Dr. Allnatt, at Frant, found in the case of the same Aurora the south-western
    part of the heavens tinged by a bright crimson band. A dark elliptical cloud
    extending from S. to S.E. was illuminated at its upper edge with a pale yellow
    light, and sent up volumes of carmine radii interspersed with green and the
    black alternating matter characteristic of elemental electricity. Almost due
    E., and of about 25 degrees elevation, was a bright insulated spot of vivid
    emerald-green, which appeared almost sufficiently intense to cast a faint
    shadow from intercepting objects. At 7 o’clock the Aurora had passed the
    zenith, and the sky presented a weird and wonderful appearance. A dark
    rugged cloud, some 8 degrees E. of the zenith, was surrounded by electric
    light of all hues—carmine, green, yellow, blood-red, white, and black; and
    the bright spot still existed in the south.</p>

    <div class="sidenote">Descriptions
    at Blackburn
    and
    Cambridge. Lapland
    Auroræ
    yellow.</div>

    <p>At Blackburn, in Lancashire, the rays were described as glowing in the
    N.E. from silvery white to deepest crimson; and at Cambridge the same
    Aurora was described as of a brilliant carmine tint. The Auroræ seen in
    Lapland by Herr Carl Bock, were, he informed me, almost invariably yellow;
    he saw only one red one.</p>

    <div class="sidenote">Hydrogen
    vacuum-tube
    suggestive
    of Aurora
    colours.</div>

    <p>The behaviour of a hydrogen Geissler vacuum-tube will be subsequently
    referred to in the Chapter on the comparison of some tubes with the Aurora
    spectrum, and is suggestive as to Aurora colours.</p>

    <div class="sidenote">Variation of
    tints in.</div>

    <p>The capillary part of this tube, when lighted by a small coil, was found
    to vary in tint—silver-white, bright green, and crimson being each in succession
    the dominant colour, according to the working of the break of the
    coil. When a spectroscope was used, the red, blue, and violet lines of the
    gas were seen to change in intensity in accordance with the light colour seen
    in the tube.</p>

    <div class="sidenote">Variation of
    colour in nitrogen
    tube
    under influence
    of magnet.</div>

    <p>A Geissler nitrogen vacuum-tube was also so arranged that the capillary part
    of it should be vertically between the conical extremities of the armatures of
    a large electro-magnet, the armatures just being clear of the outside of the
    tube. The tube was then lighted up by a small coil, and the magnet excited
    by four large double-plate bichromate cells.</p>

    <div class="sidenote">Change from
    rosy to violet
    hue.</div>

    <p>The stream of light was steady and brilliant, and, except at the violet pole,
    of the rosy tint peculiar to a nitrogen vacuum-tube. On excitation of the
    electro-magnet, the discharge was seen to diminish in volume, with an apparent
    increase in impetuosity; and not only the capillary part, but in a less
    degree the bulbs also of the tube, changed from a rosy to a well-marked violet
    hue.</p>

    <div class="sidenote">Photographic
    plates
    taken. Difference
    in.</div>

    <p>We several times connected and disconnected the magnet with its batteries,
    but always with the same result. Of the spectrum of the capillary part of
    this tube we took photographic plates with quartz prisms and lenses, taking
    care that all things should be as equal as possible, the apparatus undisturbed,
    and the time of exposure exactly the same. One plate was taken with
    the tube in its normal condition, the other while it was under the influence of
    the magnet. The spectra were identical, except that the plate of the tube
    influenced by the magnet was decidedly the brightest, and was found to
    penetrate more into the violet region (the Author’s ‘Photographed Spectra,’
    p. 60, plate xxv.). These plates effectually corroborated the change of colour,
    as the violet ray would have more photographic effect than the rosy. The
    identity of the spectra of the capillary part proved that the change in colour
    could not have proceeded from an extension of the violet glow. (A similar
    experiment will be found also detailed in Part III. Chapter XII.)</p>

    <h4 id="chap-5-3"><i>Height of the Aurora.</i></h4>

    <div class="sidenote">Height of
    Aurora. Sir John
    Franklin
    considers it
    within the
    region of the
    clouds. At no great
    elevation.</div>

    <p>Sir John Franklin (Narrative of a Journey on the Shores of the Polar Sea
    in the years 1819, ’20, ’21, ’22) says:—“My notes upon the appearance of
    the Aurora coincide with those of Dr. Richardson in proving that that phenomenon
    is frequently seated within the region of the clouds, and that it is
    dependent in some degree upon the cloudy state of the atmosphere.” And
    further:—“The observations of Dr. Richardson point particularly to the
    Aurora being formed at no great elevation, and that it is dependent upon
    certain other atmospheric phenomena, such as the formation of one or other
    of the various modifications of cirro-stratus.”</p>

    <p>Sir John Franklin also refers to notes from the Journal of Lieut. Robert
    Hood, R.N., on an Aurora:—</p>

    <div class="sidenote">Observations
    of
    Lieut. Robert
    Hood
    and Dr.
    Richardson. A beam not
    more than 7
    miles from
    the earth. An arch 7
    miles from
    the earth.</div>

    <p>The observations were made at Basquian House, and at the same time by
    Dr. Richardson at Cumberland House, quadrants and chronometers having
    been prepared for the purpose. On the 2nd April the altitude of a brilliant
    beam was 10° 0´ 0″ at 10h 1m 0s at Cumberland House. Fifty-five miles
    S.S.W. it was not visible. It was estimated that the beam was not more than
    7 miles from the earth, and 27 from Cumberland House. On the 6th April
    the Aurora was for some hours in the zenith at that place, forming a confused
    mass of flashes and beams; and in lat. 53° 22´ 48″ N., long. 103° 7´ 17″, it
    appeared in the form of an arch, stationary, about 9° high, and bearing
    N. by E. It was therefore 7 miles from the earth.</p>

    <div class="sidenote">An arch between
    6 and
    7 miles from
    the earth.</div>

    <p>On the 7th April the Aurora was again in the zenith before 10 <span class="smcapuc">P.M.</span> at
    Cumberland House, and in lat. 53° 36´ 40″ N., long. 102° 31´ 41″. The
    altitude of the highest of two concentric arches at 9h P.M. was 9°, at 9h 30m
    it was 11° 30´, and at 10h 0m 0s P.M. 15° 0´ 0″, its centre always bearing
    N. by E. During this time it was between 6 and 7 miles from the earth.
    [The bearings are true, not magnetic.]</p>

    <div class="sidenote">Sir John
    Franklin’s
    remarks.</div>

    <p>Sir J. Franklin says this was opposed to the general opinion of meteorologists
    of that period: he also noticed he had sometimes seen an attenuated
    Aurora flashing across the sky in a single second, with a quickness of motion
    inconsistent with the height of 60 or 70 miles, the least that had hitherto been
    ascribed to it.</p>

    <p></p>

    <div class="sidenote">Dr. Richardson’s
    conclusions.</div>

    <p>The needle was most disturbed, February 13, 1821, <span class="smcapuc">P.M.</span>, at a time when
    the Aurora was distinctly seen passing between a stratum of cloud and the
    earth; and it was inferred from this and other appearances that the distance
    of the Aurora from the earth varied on different nights. Dr. Richardson concludes
    that his notes prove, independent of all theory, that the Aurora is
    occasionally seated in a region of the air below a species of cloud which is
    known to possess no altitude; and is inclined to infer that the Aurora Borealis
    is constantly accompanied by, or immediately precedes, the formation of one
    or other of the forms of cirro-stratus.</p>

    <div class="sidenote">Captain
    Parry observed
    Auroræ
    near
    the Earth’s
    surface. Sir W. R.
    Grove’s observation
    at
    Chester. Mr. Ladd’s
    observation
    at Margate. The author’s
    observation
    at Kyle
    Akin, Skye.</div>

    <p>Captain Parry observed Auroræ near the earth’s surface; and records that
    he and two companions saw a bright ray of the Aurora shoot down from the
    general mass of light between him and the land, which was distant some
    3000 yards. Sir W. R. Grove (‘Correlation of Physical Forces’) saw an
    Aurora at Chester, when the flashes appeared close, so that gleams of light
    continuous with the streamers were to be seen between him and the houses—“he
    seemed to be in the Aurora.” Mr. Ladd, of Beak Street, Regent Street,
    has related to me an appearance he was struck with, and examined carefully.
    Standing in the evening in Margate Harbour, he saw a white ray of the
    Aurora, which, apparently shooting downwards, was clearly placed between
    his eye and the opposite head of the pier, which projected into the sea. Mr.
    Ladd also informed me that Prof. Balfour assured him that such an appearance
    was not unusual. In the double-arc Aurora seen by me in the Isle of
    Skye, September 11, 1874 (described <i>antè</i>, p. 23), I had a strong impression
    that the bow was near the earth, and thought that the eastern end, and some
    fleecy clouds in which it was involved, were between myself and the peaks of
    the distant mountains.</p>

    <div class="sidenote">Dalton’s calculation
    of
    100 miles. Backhouse’s
    50 to 100
    miles. Prof. Newton,
    mean
    130 miles. Elevation of
    Auroræ cannot
    exceed a
    few miles.</div>

    <p>In the article “Aurora Polaris,” Encyc. Brit., edition ix., Dalton is
    instanced as having calculated the height of an Aurora in the north of England
    at 100 miles; and Backhouse as having made many calculations, with the
    result of an average height of 50 to 100 miles. Prof. Newton, too, is quoted
    for the height of 28 Auroræ (calculated by one observation of altitude and
    amplitude of an arch) as ranging from 33 to 281 miles, with a mean of
    130 miles. It is, however, pointed out that a height of 62 miles above the
    earth’s surface would imply a vacuum attainable with difficulty, even with
    the Sprengel pump. This difficulty is then met by a reference to the observed
    altitude of some meteors, and to a suggestion of Prof. Herschel’s that electric
    repulsion may carry air or other matter up to a great height. Dr. Lardner
    (‘Museum of Science and Art,’ vol. x. p. 192) speaks of the height of Auroræ
    as not certainly ascertained; but considers them atmospheric phenomena
    scarcely above the region of the clouds, and does not think it probable that
    their elevation in any case can exceed a few miles.</p>

    <div class="sidenote">M’Clintock’s
    observations. Capt. Ross
    saw Auroræ
    on an ice-cliff,
    which
    he attributed
    to electric
    action.</div>

    <p>M’Clintock, after noticing that the beams of the Aurora were most frequently
    seen in the direction of open water, says that in some cases patches of light
    could be plainly seen a few feet above a small mass of vapour over an opening
    in the ice. Captain Ross, in his Antarctic voyage, saw the bright line of the
    Aurora forming a range of vertical beams along the top of an ice-cliff; and
    suggested this was produced by electrical action taking place between the
    vaporous mist thrown upwards by the waves against the berg, and the colder
    atmosphere with which the latter was surrounded.</p>

    <div class="sidenote">Bergman
    estimates
    height as
    468 miles.</div>

    <p>Bergman, from a mean of 30 computations, makes the height of the phenomenon
    to be 72 Swedish (about 468 English) miles.</p>

    <div class="sidenote">Boscovich
    825 miles.</div>

    <p>Father Boscovich calculated the height of an Aurora Borealis observed on
    the 16th December, 1727, to have been 825 miles.</p>

    <div class="sidenote">Mairan 600
    miles. Euler several
    thousand
    miles. Dr. Blagden
    about 100
    miles.</div>

    <p>Mairan supposed the far greater number of Auroræ to be at least 600 miles
    above the surface of the earth. Euler assigned them an elevation of several
    thousands of miles. Dr. Blagden, however, limited their height to about
    100 miles, which he supposed to be the region of fireballs—remarking that
    instances were upon record in which northern lights had been seen to join
    and form luminous balls, darting about with great velocity, and even leaving
    a train behind them like common meteors (Phil. Trans. vol. lxxiv. p. 227).</p>

    <div class="sidenote">Dalton 150
    miles.</div>

    <p>Mr. Dalton, from an observation of the luminous arches on a base of
    22 miles, found the altitude of the Aurora to be about 150 miles (Dalton’s
    ‘Meteorological Observations and Essays,’ 1793, pp. 54, 153).</p>

    <div class="sidenote">Dr. Thompson
    assumes
    considerable
    height. His table. Average of
    31 observations,
    500
    miles.</div>

    <p>Dr. Thompson, ‘Annals of Philosophy,’ vol. iv. p. 429 (1814), assumes that
    the height of the beams above the surface of the earth was much greater than
    that of most other meteorological appearances, and gives (p. 430) a table of
    Auroræ, mainly taken from Bergman, Opusc. v. p. 291, of 31 Auroræ
    observed in the years 1621 to 1793, with heights in English miles. The
    lowest is, 23rd February, 1784, London (Cavendish), 62 miles; the highest,
    23rd October, 1751, Fournerius, 1006 miles! The average of the 31 estimated
    observations gives a height of about 500 miles. It is not stated
    how these observations were obtained, though methods are mentioned how
    they might be.</p>

    <div class="sidenote">Prof. Heis’s
    instrument
    for determining
    height of
    Auroræ.</div>

    <p>Prof. Heis, of Münster, exhibited at the recent Scientific Loan Collection
    at South Kensington (‘Official Catalogue,’ 3rd edit. p. 296, No. 1231) an
    instrument for the determination of the position of the point of convergence
    of the rays of the Aurora, and for determining the height of the Aurora. A
    ball resting in a pan was to be brought into position, so that several diverging
    pencils of Aurora, when properly viewed, were covered by the rod which
    passed through the centre of the ball. The point of the rod (which could be
    moved up and down in the ball), when the instrument was set to the astronomical
    meridian, showed the azimuth and altitude of the converging point of
    the pencils of light. This point of convergence does not coincide with the
    point to which the inclination-needle directs. From the deviation of the two
    points, the height of the Aurora could be calculated.</p>

    <div class="sidenote">Professor
    Newton’s
    method of
    calculating
    height.</div>

    <p>Professor H. A. Newton (Sil. Journ. of Science, 2nd ser. vol. xxix. p. 286)
    has proposed a method of calculating the height of Auroræ by one observation
    of altitude and amplitude of an arch. It assumes that the auroral arches
    are arcs of circles, of which the centre is the magnetic axis of the earth, or at
    least that they are nearly parallel to the earth’s surface, and probably also to
    the narrow belt or ring surrounding the magnetic and astronomical poles.
    Professor Newton finds that, <i>d</i> being the distance from the observer to the
    centre of curvature of the nearest part of this belt (for England, situated about
    75° N. lat., 50° W. long.), <i>h</i> the apparent altitude of the arch, 2<i>a</i> its amplitude
    on the horizon, <i>x</i> its height, R the earth’s radius, and <i>c</i> the distance of the
    observer from the ends of the arch:—</p>

    <table summary="Equations">
      <tr>
        <td>sin φ = sin <i>d</i> cos <i>a</i> cosec(<i>d</i> + <i>h</i>)</td>
        <td class="tdr">(1)</td>
      </tr>
      <tr>
        <td>tan <i>c</i> = <i>z</i> sin <i>h</i> sin φ sec ²φ</td>
        <td class="tdr">(2)</td>
      </tr>
      <tr>
        <td><i>x</i> = R - (sec <i>c</i> - 1)</td>
        <td class="tdr">(3)</td>
      </tr>
    </table>

    <div class="sidenote">Gave a
    height from
    33 to 281
    miles, and a
    mean of 130
    miles.</div>

    <p class="noindent">This method with 28 Auroræ gave a height from 33 to 281 miles and a mean
    of 130 miles.</p>

    <p>Galle has suggested (Pogg. Ann. cxlvi. p. 133) that the height of Auroræ
    might be calculated from the amount of divergence between the apparent
    altitude of the auroral corona and that indicated by the dipping-needle, a
    principle which has been adopted in Prof. Heis’s apparatus before described.
    The results do not differ materially from Professor Newton’s.</p>

    <p>The conclusions to be arrived at from the foregoing instances and opinions
    are certainly very puzzling. The terrestrial character of some Auroræ seems
    well established. The height to which these phenomena <i>may</i> ascend is left
    almost a matter of conjecture, and further observations are very desirable.</p>

    <p></p>

    <h4 id="chap-5-4"><i>Phosphorescence.</i></h4>


    <p><span class="sidenote">Phosphorescence. Phosphorescent bands. Storm-clouds\which threw out cirri. Shone with a sort of phosphorescence. Storm-cloud surrounded by glories of a phosphorescent whiteness.</span> In the voyage of the ‘Hansa’ (‘Recent Polar Voyages,’ p. 420), on the 9th
    September, 1869, at 10 <span class="smcapuc">P.M.</span>, Aurora gleams appeared in the west, shooting
    towards the south. “Radiant sheaves and phosphorescent bands mounted
    towards the zenith,” but the phantasmagoria quickly vanished. M. Silbermann
    (‘Comptes Rendus,’ lxviii. p. 1120) mentions storm-clouds which threw out
    tufts of cirri from their tops, which extended over the sky, and resolved into,
    first, fine, and afterwards more abundant rain. (I saw a fine day example of this
    on the Lago di Guarda, ending in a copious discharge of rain attended with
    loud thunder and vivid lightning.) Usually the fibres were sinuous; but in
    much rarer cases they became perfectly rectilinear and surrounded the cloud
    like a glory, and occasionally shone <i>with a sort of phosphorescence</i>. On the
    night of 6th September, 1865, at 11 <span class="smcapuc">P.M.</span>, a stormy cloud was observed in the
    N.N.W., and lightning was seen in the dark cumulous mass. Around this mass
    extended <i>glories of a phosphorescent whiteness</i>, which melted away into the
    darkness of the starry sky. Round the cloud was a corona, and outside this
    two fainter coronæ. After the cloud had sunk below the horizon the glories
    were still visible.</p>

    <div class="sidenote">Sabine’s
    luminous
    cloud at
    Loch Scavaig,
    Skye. Other observations
    of
    luminous
    clouds.</div>

    <p>Sabine mentions a cloud frequently enveloping Loch Scavaig, in Skye, as
    being at night perfectly self-luminous, and that he saw rays, similar to those
    of the Aurora, but produced in the cloud itself. Sabine also refers to
    luminous clouds mentioned in Gilbert’s Annals, and to observations by
    Beccaria, Deluc, the Abbé Rozier, Nicholson, and Colla; and to luminous
    mists as observed by Dr. Verdeil at Lausanne in 1753, and by Dr. Robinson
    in Ireland.</p>

    <div class="sidenote">Aurora at
    Melville
    Island.</div>

    <p>He also describes (Parry’s First Voyage) an Aurora seen at Melville
    Island, and says the light was estimated as equal to that of the moon when a
    week old. Besides the pale light, <i>which resembled the combustion of phosphorus</i>,
    a slight tinge of red was noticed when the Aurora was most vivid;
    but no other colours. This Aurora was repeatedly seen <i>on the following day</i>.</p>

    <div class="sidenote">Procter
    suspects
    Aurora is
    formed in a
    mist. M’Clintock:
    Aurora is
    never visible
    in a perfectly
    clear
    atmosphere.</div>

    <p>Mr Procter, in a letter to me, suspects that the Aurora is generally formed
    in a sort of “mist or imperfect vapour;” and this mist or imperfect vapour
    seems in many instances to form part of the Aurora, and to partake of its
    self-luminous character. M’Clintock does not imagine that the Aurora is
    ever visible in a perfectly clear atmosphere. He has often observed it just
    silvering or rendering luminous the upper edge of low fog or cloud-banks,
    and with a few vertical rays feebly vibrating.</p>

    <p></p>

    <div class="sidenote">Aurora of
    Feb. 4, 1874. Illuminated
    fog-cloud. Capt. Oliver’s
    meteor-cloud. Auroral display,
    24th
    Oct., 1870. Streamers
    of phosphorescent
    cloud.</div>

    <p>An instance of apparent phosphorescence is supplied by the Aurora of the
    4th February, 1874 (<i>antè</i>), when a bright cloud of light was seen which gave
    the impression of an “<i>illuminated fog-cloud</i>.” Captain S. P. Oliver saw at
    Buncrana, Co. Donegal, on February 4, 1874, what he describes as a meteor-cloud,
    viz. “a broad band of silvery white and luminous cloud.” This
    appearance, as described by another correspondent, was evidently an imperfectly
    formed (perhaps actually forming) Auroral arc. The great Auroral
    display of the 24th of October, 1870, as seen by me, included, according to
    my notes made at the time, “streamers of opaque white phosphorescent
    cloud, very different from the more common transparent Auroral diverging
    streams of light.”</p>

    <div class="sidenote">Aurora of
    Feb. 4, 1872,
    at Frant. Radii of
    phosphorescent
    light.</div>

    <p>Describing the Aurora of February 4, 1872, at Frant, Dr. Allnatt says:—“At
    a later hour of the night the canopy of cirro-stratus had separated, and
    was transformed into luminous masses of radiant cumulus. At 10.40 the
    Aurora reappeared in the N., and sent luminous radii of white <i>phosphorescent</i>
    light from the periphery of a segment of a perfectly circular arch”&nbsp;<span class="footnote">On the occasion of the Aurora of September 24, 1870, Dr. Allnatt says, “the air seemed literally alive with the unwonted phosphorescence.”</span>.</p>

    <div class="sidenote">The author’s
    description
    of same
    Aurora. Masses of
    phosphorescent
    vapour.</div>

    <p>Again, February 4th, 1872, as described by me, the first signs of the
    Aurora were (in dull daylight) a lurid tinge upon the clouds, which suggested
    the reflection of a distant fire; while scattered among these, “torn and broken
    masses of white vapour having a phosphorescent appearance” reminded me
    of a similar observation in October 1870.</p>

    <div class="sidenote">Day Auroræ
    must have a
    phosphorescent
    glow. Ångström
    considers
    yellow-green
    line
    due to fluorescence
    or
    phosphorescence. Oxygen and
    some of its
    compounds
    phosphorescent.</div>

    <p>The day Auroræ, which are elsewhere described, and are not very uncommon,
    could, we may presume, hardly be seen without the presence of
    some phosphorescent glow. Professor Ångström, in his Aurora Memoir
    (discussed elsewhere), in discussing the yellow-green line, considers the only
    probable explanation to be that it owes its origin to fluorescence or phosphorescence.
    He says that some fluorescence is produced by the ultra-violet
    rays; and adds, “an electric discharge may easily be imagined, which, though
    in itself of feeble light, may be rich in ultra-violet light, and therefore in a
    condition to cause a sufficiently strong fluorescent light.” And he refers to
    the fact that oxygen and some of its compounds are phosphorescent.</p>

    <div class="sidenote">A phosphoretted
    hydrogen
    spectrum-band
    is close
    to yellow-green
    auroral line. Phosphorescent
    or fluorescent
    after-glow
    of electric
    discharge.</div>

    <p>In the examination of certain spectra connected with the Aurora, detailed
    in Part II., I have shown that the bright edge of one of the phosphoretted
    hydrogen bands is in close proximity to the yellow-green Auroral line. I
    have also referred to the peculiar brightening by reduction of temperature of
    one of the bands in the red end of the spectrum of phosphoretted hydrogen,
    so that from almost invisible it became bright, and to the peculiar brightening
    of a line in the yellow-green in certain “Aurora” and phosphorescent tubes.
    It has also been observed that the electric discharge has a phosphorescent or
    fluorescent after-glow (isolated, I believe, by Faraday). It seems difficult to
    avoid in some way connecting all these circumstances with the yellow-green
    line of the Aurora, if not also with the line in the red.</p>

    <div class="sidenote">Sorby’s experiments
    on fluorescence
    and
    absorption. Bonelleine,
    spectrum of. Coloured
    layer of
    fungi. Spectrum of
    <i>Oscillatoriæ</i>.</div>

    <p>Mr. Sorby, in his experiments on the connexion between fluorescence and
    absorption (‘Monthly Microscopical Journal’), found in the spectrum of a
    solution in alcohol of a strongly fluorescent substance called bonelleine (the
    green colouring-matter found in the <i>Aurelia Bonellia-viridis</i>) two bright
    bands, the one red and the other green, with centres respectively at 6430
    and 5880, and their limits towards the blue end at 6320 and 5820. On
    adding an acid the red band changed its place to 6140. The superficial
    membranous coloured layer of the fungi <i>Russula nitida</i> and <i>vesca</i> in alcohol
    gave an absorption band with centre at 5540, while the spectrum of fluorescence
    extended to 4400. A solution of <i>Oscillatoriæ</i> in water gave a
    spectrum of absorption with bands at 6200 and 5690; while the spectrum of
    fluorescence showed two bright bands having their centres at 6470 and 5800,
    and their limits towards the blue end at 6320 and 5710.</p>

    <div class="sidenote">Sea phosphorescence,
    a continuous
    spectrum.</div>

    <p>These instances of course cannot be connected with the Aurora except as
    showing the spectrum region and lines of fluorescence. The sea phosphorescence,
    according to Professor Piazzi Smyth, has a continuous spectrum
    extending from somewhat below E to near F (Plate V. fig. 3).</p>

    <div class="sidenote">Ångström
    finds the
    sky almost
    phosphorescent.</div>

    <p>Ångström, on the occasion of the starry night when he found traces of the
    green line in all parts of the heavens, speaks of the sky as being “almost
    phosphorescent.”</p>

    <div class="sidenote">Author of
    article in
    Encyc. Brit.
    suggests
    that the
    phosphorescent
    or fluorescent
    light
    may be due
    to chemical
    action. Herschel’s
    observation
    of phosphorescence
    in
    Geissler and
    “garland”
    tubes.</div>

    <p>The author of the Aurora article in the Encyc. Brit. suggests that the
    phosphorescent or fluorescent light attributed to the Aurora may be due to
    chemical action. He also questions Ångström’s assumption that water-vapour
    is absent in the higher atmosphere, and thinks that it and other
    bodies may, by electric repulsion, be carried above the level they would
    attain by gravity. He then continues that if discharges take place between
    the small sensible particles of water or ice in the form of cirri (as Silbermann
    has shown to be likely) surface decomposition would ensue, and it is
    highly probable the nascent gases would combine with emission of light.
    He adds “that it has been almost proved that in the case of hydrogen
    phosphide the very characteristic spectrum (light?) produced by its combustion
    is due neither to the elements nor to the products of combustion, but
    to some peculiar action at the instant of combination; and it is quite possible
    that under such circumstances as above described water might also give
    an entirely new spectrum.” Professor Herschel has referred to the phosphorescent
    light which remains glowing in Geissler tubes after the spark has
    passed, and to the fact that one of the globes of a “garland” tube which
    was heated did not shine after the spark had passed, apparently because of
    the action of heat on the ozone to which the phosphorescence might be due.
    (See experiments on Mr. Browning’s bulbed tube, Part III. Chap. XV.)</p>

    <h4 id="chap-5-5"><i>Aurora and Ozone.</i></h4>

    <div class="sidenote">Aurora and
    Ozone. Smells of
    sulphur
    during
    Auroræ attributed
    to
    ozone.</div>

    <p>Accounts are given by travellers in Norway of their being enveloped in the
    Aurora, and perceiving a strong smell of sulphur, which was attributed to the
    presence of ozone. M. Paul Rollier, the aëronaut, descended on a mountain
    in Norway 1300 metres high, and saw brilliant rays of the Aurora across a
    thin mist which glowed with a remarkable light. To his astonishment, an
    incomprehensible muttering caught his ear; when this ceased he perceived a
    very strong smell of sulphur, almost suffocating him (‘Arctic Manual,’ p. 726).</p>

    <div class="sidenote">Question
    whether the
    oxygen of
    the air may
    be changed
    into ozone.</div>

    <p>In the case of the Aurora, the question naturally arises whether the
    oxygen of the air may be changed into ozone, perhaps also whether the
    nitrogen may not be modified in some similar manner.</p>

    <div class="sidenote">Ozone destroyed
    by
    heat.</div>

    <p>The absorption spectra of oxygen, and of the same gas in its form of ozone,
    may possibly differ; but this can hardly happen in the case of incandescent
    oxygen, for ozone is at once destroyed by heat at 300°, and slowly at 100°,
    and must be partially at least destroyed by the heat of the discharge. If
    any lines were due to ozone in such a spectrum, we should expect they would
    be weakened by heat and brightened by cold.</p>

    <div class="sidenote">Ozone in a
    large bell-receiver
    not
    manifested
    in spectrum.</div>

    <p>In the case of a continued discharge in a large exhausted bell-receiver, the
    presence of ozone in considerable quantities was manifested to us by its
    odour when the receiver was removed from the pump; but the spectrum of
    the stream of light did not appear to differ from that in Geissler tubes.</p>

    <div class="sidenote">Professor
    Dewar demonstrates
    that ozone
    is condensed
    oxygen.</div>

    <p>In a course of lectures at the Royal Institution in March 1878, on the
    Chemistry of the Organic World, Prof. Dewar appears to have demonstrated,
    by Prof. Andrews’ apparatus, that ozone is really condensed oxygen, and,
    further, that during this condensation heat is absorbed, which is evolved
    during the decomposition or re-expansion.</p>

    <div class="sidenote">Refers to the
    silent discharge
    between
    the atmosphere
    and
    the earth.</div>

    <p>He also exhibited the oxidizing power of ozone in its action on mercury,
    and commented on its similar action upon organic matter in forming nitrates,
    and on its remarkable bleaching properties, but added there was as yet no
    proof of its combining with free nitrogen. That peroxide of hydrogen
    accompanies the formation of ozone by the slow combustion of phosphorus,
    and that this peroxide acts with ozone in decomposing organic bodies, though
    in an inexplicable manner, the Professor considered to be proved. He also
    referred to the silent discharge probably perpetually going on between the
    upper and lower strata of the atmosphere, and also between these and the
    earth, accounting, as the Professor considered, for some of the chemical
    actions whereby nitrogenous compounds are formed in the soil.</p>

    <div class="sidenote">No spectrum
    of
    ozone obtained.</div>

    <p>As far as I am aware, no information as to a possible spectrum of ozone,
    or a modification of the oxygen or other spectra by its presence, has, up to
    the present time, been obtained&nbsp;<span class="footnote">See, however, Dr. Schuster’s article “On the Spectra of Lightning,” Phil. Mag. May 1879, p. 316.</span>.</p>

    <div class="sidenote">Suggestion
    to subject
    electric discharge
    to
    influence of
    cold.</div>

    <p>It has been suggested by Mr. Procter and myself that the electric discharge
    in an exhausted moist tube, if subjected to a considerable degree of cold,
    might produce a modification of the air-spectrum, perhaps even a spectrum
    analogous to that of the Aurora.</p>

    <p>For some further notes on this subject see Appendix D (Aurora and Ozone).</p>

    <h4 id="chap-5-6"><i>Polarization of the Aurora Light.</i></h4>

    <div class="sidenote">Polarization
    of the Aurora
    light. Mr. Ranyard
    found none.</div>

    <p>In ‘Nature,’ vol. vii. p. 201, is contained an account of observations of the
    polarization of the zodiacal light and of the Aurora, by Mr. A. Cowper
    Ranyard, who, using both a double-image prism and a Savart on the great
    Aurora of February 4th, 1872, detected no trace of polarization. He also
    examined a smaller one of 10th November, 1871, with a like result.</p>

    <div class="sidenote">Prof. Alexander
    found
    strong polarization
    in
    latitude 60°.</div>

    <p>Mr. Fleming (who refers to these observations) remarks that the only other
    account he had met with was contained in Prof. Stephen Alexander’s Report
    on his Expedition to Labrador, given in Appendix 21 of the U.S. Coast Survey
    Report for 1860, p. 30. Professor Alexander found strong polarization with
    a Savart’s polariscope, and thought that the dark parts of the Aurora gave
    the strongest polarization. This was in latitude about 60°, at the beginning
    of July, and near midnight. It is not stated whether there was twilight or
    air-polarization at the time, nor is the plane of polarization given.</p>

    <div class="sidenote">Mr. Shroeder
    found
    no polarization.</div>

    <p>The question naturally arises, especially as the darkest parts of the Aurora
    are usually situated low down near the horizon, whether the polarization in
    the latter case did not proceed from the atmosphere and not from the Aurora
    itself. Mr. Shroeder found no traces of polarization in the Aurora of
    February 4th, 1872. Further examinations of the Aurora with some delicate
    form of polariscope would seem very desirable.</p>

    <div class="sidenote">Polarization
    not found in
    the zodiacal
    light; except
    faint
    traces by
    Mr. Burton.</div>

    <p>The evidence of polarization in the case of the zodiacal light seems also
    almost entirely negative—Mr. Ranyard pointing out observations of his own,
    of Captain Tupman, and of Mr. Lockyer with this result. Mr. Burton,
    using a Savart set so as to give a black centre when the bands were parallel
    to the plane of polarization, believed he detected faint traces of polarization
    in the brightest parts of the zodiacal light (as seen in Sicily), the bands being
    black-centred when their direction coincided with the axis of the cone of
    light. Mr. Burton saw no trace of bands when examining the slight remaining
    twilight apart from the zodiacal light. Mr. Ranyard was not able to
    confirm Mr. Burton’s observations on the same evening and with the same
    instrument.</p>

    <h4 id="chap-5-7"><i>Number of Auroræ.</i></h4>

    <div class="sidenote">Number of
    Auroræ. Sir John
    Franklin’s
    observations.</div>

    <p>Sir John Franklin saw in the Arctic Regions, in the years 1819, 1820,
    1821, 1822:—In the month of September two Auroræ, in October three, in
    November three, in December two, in January five, in February seven, in
    March sixteen, in April fifteen, and in May eleven.</p>

    <div class="sidenote">Periodicity
    as to days
    not established.</div>

    <p>Periodicity as to days seems to have no certain law; and though certain
    days in February and March are marked as those of fine returning displays,
    they must be looked on as accidental.</p>

    <div class="sidenote">Maxima and
    minima.</div>

    <p>Two well-marked annual maxima seem to occur in March and October (the
    latter the greater), and two minima in June and January, the greater in
    June (Encyc. Brit.). The 4th of February, 1872, and same day 1874, are,
    however, curious instances of a recurring remarkable display.</p>

    <div class="sidenote">Kæmtz’s
    table.</div>

    <p>A table by Kæmtz, showing the number of Auroras in each month of
    the year, with the maxima and minima as above stated, will be found on
    Plate V. fig. 5.</p>

    <div class="sidenote">Dr. Hayes’s
    observations
    in winter of
    1860-61.</div>

    <p>Dr. Hayes has observed that in the winter of 1860-61 (when the ten or
    eleven years’ inequality was at its maximum) only three Auroræ were seen and
    recorded, and they were feeble and short in duration.</p>

    <div class="sidenote">Captain
    Maguire’s
    observations
    at Point
    Barrow as
    to number
    and time of
    appearances.</div>

    <p>Captain Maguire, at Point Barrow (1852-54), reports that the Aurora was
    seen six days out of seven, and on 1079 occasions, being nearly one third of
    the hourly observations. It was seldom seen between 9 <span class="smcapuc">A.M.</span> and 5 <span class="smcapuc">P.M.</span>, not
    at all between 10 <span class="smcapuc">A.M.</span> and 4 <span class="smcapuc">P.M.</span> It increased regularly and rapidly from
    5 <span class="smcapuc">P.M.</span> until 1 <span class="smcapuc">A.M.</span>, and then diminished in the same way until 9 <span class="smcapuc">A.M.</span></p>

    <p>The winters of 1877 and 1878 and the springs of 1878 and 1879 have been
    singularly deficient in Auroræ. I have seen none at Guildown.</p>

    <p></p>

    <h4 id="chap-5-8"><i>Duration of Aurora.</i></h4>

    <div class="sidenote">Duration of
    Aurora. Sometimes a
    few minutes;
    at other
    times the
    whole night
    or even days.</div>

    <p>In the article in the ‘Edinb. Encyc.’ before referred to some remarks are
    made on the duration of the Aurora. Sometimes it is formed and disappears
    in the course of a few minutes. At other times it lasts for hours or during
    the whole night, or even for two or three days together. Musschenbroek
    observed one in 1734 which he considered to have lasted ten days and nights
    successively, and another in 1735 which lasted from the 22nd to the 31st
    March.</p>

    <div class="sidenote">Auroræ may
    run on into
    the day
    without
    being
    noticed.</div>

    <p>With respect to Captain Maguire’s observations (<i>antè</i>) it may be remarked
    that Auroræ may doubtless frequently run on into and through the day
    without their being noticed (instances, however, are known of Auroræ seen
    in daylight); and hence it is difficult to judge of the limit of duration of a
    particular Aurora unless indications are sought for during the day (by the
    shapes of clouds, action of the magnet, &amp;c.) as well as during the night.
    Probably Auroræ seen during successive nights may be parts of a continuous
    discharge.</p>

    <h4 id="chap-5-9"><i>The Travelling of Auroræ.</i></h4>

    <div class="sidenote">Travelling
    of Auroræ. Donati’s investigations.</div>

    <p>Donati undertook to study the Aurora with reference to the mode of its
    extension; and he arrived at the result that the Aurora of February 4, 1872,
    was not observed in different regions of the earth in the same physical
    moment; <i>but everywhere at the same local hour</i>, as in the case of celestial
    phenomena, which do not share in the earth’s rotation.</p>

    <div class="sidenote">Questions
    sent to
    Italian Consuls.</div>

    <p>The Minister of Foreign Affairs sent a circular to all Italian Consuls,
    asking them the necessary questions; and in reply received reports from
    forty-two places in our hemisphere and from four in the southern, the
    places embracing in one latitude the considerable extent of 240 degrees of
    longitude.</p>

    <p>An epitome of the tables (in which the results are divided into three zones)
    is as follows:—</p>

    <div class="sidenote">Table of
    results.</div>

    <table summary="Results" class="borders">
      <tr>
        <th>Zone.</th>
        <th>Mean longitude<br />of zone.</th>
        <th>No. of<br />stations.</th>
        <th>Mean hour<br />of maximum.</th>
        <th>Mean hour<br />of end.</th>
      </tr>
      <tr>
        <td class="bl">Eastern</td>
        <td class="nw">2 hrs.  5 mins. E.</td>
        <td class="tdr">9</td>
        <td class="tdr">9½ hrs.</td>
        <td class="tdr">12¼ hrs.</td>
      </tr>
      <tr>
        <td class="bl">Middle</td>
        <td class="nw">0 hr.  20 mins. E.</td>
        <td class="tdr">17</td>
        <td class="tdr">8½ hrs.</td>
        <td class="tdr">11½ hrs.</td>
      </tr>
      <tr>
        <td class="bb bl">Western</td>
        <td class="bb nw">5 hrs. 38 mins. W.</td>
        <td class="bb tdr">13</td>
        <td class="bb tdr">8¾ hrs.</td>
        <td class="bb tdr">9¾ hrs.</td>
      </tr>
    </table>

    <p></p>

    <div class="sidenote">Extensions
    of the
    Aurora. The Aurora
    passed
    through
    four periods.
    First period
    of origin,
    light weak.
    Second
    period, increase
    of intensity.
    Third period,
    continuous
    brightness.
    Fourth
    period, decrease.</div>

    <p>Donati summed up the facts:—That the light phenomena of this Aurora
    began to show themselves in the extreme east of the southern hemisphere in
    Eden and Melbourne; shortly after, they were observed in the east of our
    hemisphere in China (but not in Japan); from China the Aurora passed
    over the whole of Asia and Europe, and crossed the Atlantic and the
    American Continent as far as California. It was invisible in Central and
    South America. During these immense extensions it passed through four
    periods. In the first (called by Donati the period of origin) the light of the
    Aurora was pretty weak, and spread from Shanghai to Bombay; in the
    second period, during which it passed on from Bombay to Taganrog, it
    acquired a sudden increase of intensity; in the third period (called by Donati
    the normal) the Aurora passed over Europe from east to west with regularity
    and a continuous brightness; the fourth period, that of decrease, was observed
    in America. The Aurora had a tendency to end earlier in reference
    to the local hour in the western stations than in the eastern. The acceleration
    on an average of the end of the phenomenon was twenty minutes for
    every hour of longitude.</p>

    <div class="sidenote">Donati’s
    conclusions. Explanation
    of mode of
    propagation
    of same
    Aurora.</div>

    <p>Donati concluded that these facts were not reconcilable with the theory of
    the Aurora depending on meteorological and electro-magnetic phenomena of
    the globe. Since, too, we have not a yearly, but a ten-yearly period of the
    Aurora, which coincides with that of sun-spots and terrestrial magnetism,
    Donati supposed that the cosmic causes of the polar lights were electro-magnetic
    currents between the sun and the earth. This would explain the
    mode of propagation of the Aurora of 4th February. Conceive an electric
    current going from the earth to the sun, or <i>vice versâ</i>; certain phenomena
    of the Aurora could only be observed in those parts of the atmosphere which
    have a determinate position or direction with reference to this current; and
    consequently these phenomena would be successively visible on the different
    meridians, as these meridians, by reason of the earth’s rotation, assume the
    same position to the current. For the Aurora to be visible certain meteorological
    and telluric circumstances must, however, doubtless work together
    with the cosmical cause.</p>

    <h4 id="chap-5-10"><i>Geographical Distribution of Auroræ (Fritz and Loomis).</i></h4>

    <div class="sidenote">Geographical
    Distribution
    of
    Auroræ. Prof. Fritz’s
    and Prof.
    Loomis’s
    line of frequency.</div>

    <p>Professors Fritz and Loomis have investigated this subject; and Petermann’s
    ‘Mittheilungen,’ vol. xx. (1874), contains a paper by the former,
    from which it appears that the northern limit of Auroræ chosen by Professor
    Loomis nearly coincided, except in England, with a line of frequency in
    Professor Fritz’s paper. This line nearly passes through Toronto, Manchester,
    and St. Petersburg. Professor Loomis places it as far north as
    Edinburgh. On a line across Behring’s Straits, and coming down below
    60° N. in America and the Atlantic, and just north of the Hebrides, to
    Dröntheim, and including the most northern points of Siberia, the frequency
    is represented by 100.</p>

    <div class="sidenote">Within this
    another zone
    of greatest
    frequency
    and intensity.</div>

    <p>Within this is another zone of greatest frequency and intensity, which
    passes just south of Point Barrow, in lat. 72° N., on the northern coast of
    America, and by the Great Bear Lake to Hudson’s Bay, where it reaches a
    latitude of 60°, then on to Nain, on the coast of Labrador, and to the south
    of Cape Farewell; then bending sharper to the northward, it passes between
    Iceland and the Faroe Islands, near to the North Cape, on by the northern
    ice-sea to Nova-Zembla and Cape Tschejuskin, and on just to the north of
    the Siberian coast to the south of Kellett Land, thence returning to Point
    Barrow.</p>

    <div class="sidenote">Lines on
    which annually
    nearly
    the same
    number of
    Auroræ are
    seen.</div>

    <p>More or less parallel with this line are the lines on which annually nearly
    the same number of Auroræ are seen. The line for one Aurora annually
    went from Bordeaux, through Switzerland, past Krakau, south of Moscow
    and Tobolsk, to the northern end of Lake Baikal, on to the Sea of Ochotsk
    and to the Southern Aleutes, thence through Northern California to the
    mouth of the Mississippi and to Bordeaux. The line for five Auroræ
    annually went from Brest through Belgium, Stettin, Wologda, between
    Tobolsk and Beresow, parallel to the previous line to Ochotsk, and on to
    Brest, &amp;c. Almost exactly with the line of greatest frequency coincides
    the line forming the boundary of the direction of visibility of the Northern
    Light towards the Pole or towards the Equator; while northwards of this
    line the Polar Light is seen in the direction towards the Equator; and from
    all stations the Northern Lights are seen in directions which are pretty much
    normal to that curve and the entire system of isochasms.</p>

    <div class="sidenote">Assumed
    connexion
    between
    Aurora
    and ice-formation.</div>

    <p>Professor Fritz has remarked that the curves of greater frequency tend
    towards the region of atmospheric pressure, and also that they bear some
    relation to the limit of perpetual ice—tending most southward where, as in
    North America, the ice limit comes further south. He also endeavours to
    show a connexion between the periods of maximum of Auroræ and those of
    ice-formation, and considers ice to be an important local cause influencing
    their distribution. These being most frequently seen over open water in
    the Arctic regions, has been referred to as noticed by Franklin and others.</p>

    <p></p>

    <h4 id="chap-5-11"><i>Extent and principal Zone of the Aurora.</i></h4>

    <div class="sidenote">Extent and
    principal
    zone of
    Aurora. M. Moberg’s
    Finland observations
    (1846-55)
    compared by
    Prof. Fritz
    with those
    in other regions.</div>

    <p>The Finland observations, published by M. Moberg in his ‘Polarlichter
    Katalogue’ of Northern Lights in the years 1846-55, numbering 1100, have
    been compared by Prof. Fritz, in his paper in the ‘Wochenschrift für Astronomie,’
    with the auroral phenomena of the same period in all other regions.
    The Table shows that of 2035 days of the months August to April on
    which Northern Lights were seen, 1107 days were those of Northern Lights
    for Finland. On 794 they were visible simultaneously in America, and
    mostly also in Europe; on 101 days in Europe only, and on 212 days in Finland
    only. On 958 days Northern Lights were visible in Europe and America
    which were not visible in Finland. All these numbers refer only to the
    months August to April, as in the remaining months the brightness of the
    night in Finland makes such observations impossible.</p>

    <p>The conclusion is arrived at that a large portion of Auroræ have no very
    great extension, or that the causes producing the phenomena must often be
    of a very local character; while in another portion of the phenomena the
    extent, or the regions of simultaneous appearance are very considerable.</p>

    <div class="sidenote">Number
    limited to
    Finland
    only small.</div>

    <p>The number limited to Finland, for which hitherto corresponding observations
    from other lands are wanting, is very small—212, or 19 per cent. of the
    whole number seen in Finland. With the increase of frequency of the
    phenomena at the time of maximum, the number observed in Finland and
    America on the same day increases; while those observed in Finland and
    Europe only, or in Finland only, decreased, in accordance with the known
    law that with the frequency the intensity and extent also increase.</p>

    <div class="sidenote">One third of
    Auroræ seen
    in America
    and Europe
    simultaneously.</div>

    <p>Between 1826 and 1855, of 2878 days on which, in America, the Northern
    Lights were seen, there were 1065 on which they were also visible in Europe;
    so that at least every third day of Auroræ was common to both these portions
    of the globe. In the years 1846 to 1855, and 1868 to 1872, there were in
    the first period 657 Northern-Light days common to America and Europe out
    of 1691, and in the second 397 out of 715.</p>

    <div class="sidenote">Local occurrence
    of
    the Aurora
    not in favour
    of its assumed
    cosmical
    nature.</div>

    <p>The comparison by Prof. Fritz of M. Moberg’s Finland observations has
    been lately reviewed in ‘Nature’ (March 8, 1878) and the result arrived at
    that, “After ten years, in spite of the vastly accumulated material of careful
    observations, there appears no necessity to change Herr Fritz’s system of
    curves in any essential detail; indeed certain parts of the same, which were
    at first only based on probability and supposition (the part of the principal
    zone between the north of Norway and Nishen Kolynisk as an instance), we
    now know with perfect certainty to be correct.” It has been remarked that
    the local occurrence of Auroræ is not in accordance with the hypothesis of
    the phenomenon being one of a cosmical nature.</p>

    <p>The winter of 1870 was remarkable for brilliant displays; and the displays
    of October 24th and 25th, 1870, were remarkably brilliant in England and
    in America also, and the Aurora Australis was seen on the same days at
    Madras. These displays were seen in England and America in the daytime
    as patches or coronæ of white light, with streamers stretching upward
    from them.</p>

  </section>

  <!-- Chapitre 6_________________________________________________________-->
  <section class="chapter" id="chapter-6">
    <h3 class="titlechapter" id="chap-6">The Aurora in connexion with other Phenomena</h3>
    <p class="shorter">The Aurora in connexion with other Phenomena</p>

    <h4 id="chap-6-1"><i>Auroræ and Clouds.</i></h4>

    <div class="sidenote">Auroræ and
    clouds. Dr. Richardson’s
    observations. Aurora constantly
    accompanied
    by or immediately
    precedes
    the
    formation
    of cirro-stratus.</div>

    <p>Dr. Richardson (‘Sir John Franklin’s Narrative’), so long ago as the years
    1819-1822, made many recorded observations on the connexion of clouds
    with the Aurora Borealis in the Polar regions. Some of these are alluded
    to in Chapter V., section “Height of the Aurora,” for the purpose of showing
    the moderate distance he found it to be above the earth; and his inference is
    there mentioned, “that the Aurora Borealis is constantly accompanied by or
    immediately precedes the formation of one or other of the various kinds of
    cirro-stratus.” On the 13th November and 18th December, 1820, the connexion
    of an Aurora with a cloud intermediate between cirrus and cirro-stratus
    is mentioned. It is, however, also mentioned that the most vivid
    coruscations of the Aurora were observed when there were only a few
    attenuated shoots of cirro-stratus floating in the air, or when that cloud was
    so rare that its existence was only known by the production of a halo round
    the moon. (An instance of attenuated streaks of cirro-stratus in connexion
    with an auroral arc will be found in the Aurora seen at Guildown on the 4th
    February 1874, a sketch of which is reproduced on Plate VI. fig. 1.)</p>

    <div class="sidenote">Polarity discerned
    in
    cirro-stratus
    clouds.</div>

    <p>Dr. Richardson goes on to express his opinion that he, on some occasions,
    discerned a polarity in the masses of clouds belonging to a certain kind of
    cirro-stratus (approaching cirrus), by which their long diameters, having all
    the same direction, were made to cross the magnetic meridian nearly at right
    angles.</p>

    <div class="sidenote">Apparent
    polarity of
    Aurora
    might perhaps
    be
    ascribed to
    the clouds
    themselves.</div>

    <p>Dr. Richardson further suggests that if it should be thereafter proved that
    the Aurora depends upon the existence of certain clouds, its apparent polarity
    might perhaps be ascribed to the clouds themselves which emit the light; or,
    in other words, the clouds might assume their peculiar arrangement through
    the operation of one cause (magnetism, for instance), while the emission of
    light might be produced by another—a change in their internal constitution
    perhaps connected with a motion of the electric fluid.</p>

    <p>Dr. Richardson further remarks that, generally speaking, the Aurora
    appeared in small detached masses for some time before it assumed that convergency
    towards the opposite parts of the horizon which produced the arched
    form.</p>

    <div class="sidenote">Sir John
    Franklin’s
    observations.</div>

    <p>Sir John Franklin says in his Polar expeditions he often perceived the clouds
    in the daytime disposed in streams and arches such as the Aurora assumes.</p>

    <span class="sidenote">Dr. Low’s.</span>

    <p>Dr. Low (‘Nature,’ iv. p. 121) considers he witnessed a complete display of
    auroral motions in cirrus cloud, and considers all clouds subject to magnetic
    or diamagnetic polarization; he states that when the lines converge towards
    the magnetic pole fine weather follows, and when at right angles to it wet and
    stormy.</p>

    <div class="sidenote">M. Silbermann’s
    observations,
    15th April,
    1869. Cirrus
    clouds took
    the place of
    the Aurora.</div>

    <p>In the Encyc. Brit. edition 9, article “Aurora Polaris,” after referring to
    the evidence of Franklin, Richardson, and Low, M. Silbermann (‘Comptes
    Rendus,’ lxviii. p. 1051) is quoted in detail for observed connexion between
    the Aurora and cirrus cloud. 15th April, 1869, at 11h 16m, an Aurora
    appeared and disappeared; but it seemed as if the columns were still visible,
    and it soon became obvious that fan-like cirrus clouds, with their point of
    divergence in the north, had taken the place of the Aurora. Between 1 and
    2 <span class="smcapuc">A.M.</span> the clouds had passed the zenith, and let fall a little fine frozen rain.
    At 4 <span class="smcapuc">A.M.</span> the cirrus of the false Aurora was still visible, but deformed towards
    the top, and presenting a flaky aspect. The cirrus never appeared to replace
    the Aurora either from right or left, but to substitute itself for it like the
    changes of a dioramic view.</p>

    <div class="sidenote">Payer
    thinks the
    transition of
    Aurora into
    clouds not
    proved.</div>

    <p>Payer, in his ‘Austrian Arctic Voyages,’ thinks that the occurrence of the
    Aurora during the day (i. e. <i>light clouds with its characteristic movement</i>) had
    been rather imagined than actually observed, and that the transition of white
    clouds into auroral forms at night has never been satisfactorily proved. He,
    however, mentions the mist-like appearance of the Aurora.</p>

    <div class="sidenote">Dr. Allnatt’s
    observations,
    4th
    February,
    1872, at
    Frant. Aurora
    passed into
    cirro-stratus.</div>

    <p>Dr. Allnatt observed the splendid Aurora of 4th February, 1872, at Frant,
    and noticed the weird and wonderful appearance of the phenomena. At 6 <span class="smcapuc">P.M.</span>
    the Aurora commenced by the S.W. portion of the heavens being tinged with
    a bright carmine hue, and in a short time the whole visible hemisphere was
    lighted up. A dark elliptical cloud extending from S. to S.E. and S.W. sent
    up volumes of coloured radii. At 7 the Aurora had passed the zenith, and a
    dark, broken, rugged cloud some 8° E. of zenith was surrounded by electric
    light of all hues. At 7.40 the Aurora began to wane, and passed into a
    homogeneous cirro-stratus of sufficient density to obscure the stars, disappearing
    at 7.45.</p>

    <div class="sidenote">Later, cirro-stratus
    was transformed
    into luminous
    cumulus.</div>

    <p>At a later hour of the night the canopy of cirro-stratus had separated and was
    transformed into luminous masses of radiant cumulus; so that, as Dr. Allnatt
    observes, there were called in requisition almost all the most prominent cloud-modifications
    during the progress of the phenomena. The succession of
    formation, transformation, and reformation from Aurora to cloud and from
    cloud to Aurora was, Dr. Allnatt concluded, conclusive of the theory before
    advanced of the electric origin of the recurrent rayed cloud-modifications in
    the place of the magnetic meridian, over which so much mystery had been
    cast.</p>

    <h4 id="chap-6-2"><i>Aurora and Thunder-storms.</i></h4>

    <div class="sidenote">Aurora and
    thunder-storms.
    Silbermann’s
    theory.</div>

    <p>Silbermann asserts that Auroræ are produced by the same general phenomena
    as thunder-storms, and concludes that the Auroræ of 1859 and 1869 assumed
    the character of thunder-storms which, instead of bursting in thunder, had
    been drawn into the upper parts of the atmosphere, and their vapour being
    crystallized in tiny prisms by the intense cold, the electricity became luminous
    in flowing over these icy particles.</p>

    <div class="sidenote">Prof. Piazzi
    Smyth on
    monthly
    frequency of
    Auroræ and
    storms.</div>

    <p>Professor Piazzi Smyth has observed that the monthly frequency of Auroræ
    varies inversely with that of thunder-storms. His Table of comparisons is as
    follows:—</p>

    <div class="sidenote">His table
    of observations.</div>

    <table summary="Piazzi Smyth's observations">
      <tr>
        <th class="smaller">Month.</th>
        <th class="smaller">Lightning.</th>
        <th class="smaller">Auroræ.</th>
      </tr>
      <tr>
        <td>January</td>
        <td class="tdr">24·0</td>
        <td class="tdr">29·7</td>
      </tr>
      <tr>
        <td>February</td>
        <td class="tdr">14·4</td>
        <td class="tdr">42·5</td>
      </tr>
      <tr>
        <td>March</td>
        <td class="tdr">7·0</td>
        <td class="tdr">35·0</td>
      </tr>
      <tr>
        <td>April</td>
        <td class="tdr">15·4</td>
        <td class="tdr">27·5</td>
      </tr>
      <tr>
        <td>May</td>
        <td class="tdr">37·4</td>
        <td class="tdr">4·8</td>
      </tr>
      <tr>
        <td>June</td>
        <td class="tdr">48·0</td>
        <td class="tdr">0·0</td>
      </tr>
      <tr>
        <td>July</td>
        <td class="tdr">55·2</td>
        <td class="tdr">0·5</td>
      </tr>
      <tr>
        <td>August</td>
        <td class="tdr">38·4</td>
        <td class="tdr">12·6</td>
      </tr>
      <tr>
        <td>September</td>
        <td class="tdr">22·4</td>
        <td class="tdr">36·6</td>
      </tr>
      <tr>
        <td>October</td>
        <td class="tdr">20·8</td>
        <td class="tdr">49·4</td>
      </tr>
      <tr>
        <td>November</td>
        <td class="tdr">15·0</td>
        <td class="tdr">32·4</td>
      </tr>
      <tr>
        <td>December</td>
        <td class="tdr">15·0</td>
        <td class="tdr">28·8</td>
      </tr>
      <tr>
        <td>Mean of whole year</td>
        <td class="tdr bt">24·0</td>
        <td class="tdr bt">20·1</td>
      </tr>
    </table>

    <div class="sidenote">Silbermann’s
    observations
    15th April,
    1869.
    30th April,
    1865.</div>

    <p>Silbermann, on 15th April, 1869, observed a fall of rain (tiny crystals of ice)
    on the disappearance of an Aurora and its change into cloud forms (see section,
    “Auroræ and Clouds,” p. 53). He also observed a rain of little sparkling ice-prisms
    on 30th April, 1865, at Paris, the city being then enveloped in a cirrus
    of vertical fibres similar to that which frequently accompanies the Aurora.</p>

    <p></p>

    <p>On the occasion of the Aurora seen by me at Guildown, 4th February,
    1872, rain fell immediately succeeding the formation of the corona.</p>

    <p>The falling of rain as an immediate sequence of an Aurora seems, however,
    to be rather the exception than the rule; but possibly this may vary with the
    character of the Aurora itself—whether it be of the crimson class, passing
    into cloud and accompanied with much electric disturbance, or of the more
    quiescent white.</p>

    <div class="sidenote">A falling
    barometer
    observed to
    follow
    Auroræ.</div>

    <p>A falling barometer following a display of Auroræ has been noticed by Sir
    John Franklin and others; and in some cases (notably one in Sicily before
    referred to) storms and floods have accompanied this.</p>

    <div class="sidenote">Professor
    Christison’s
    observations.</div>

    <p>In a paper read before the Royal Society of Edinburgh in 1868, Prof.
    Christison mentioned, as a fact of importance to agriculturists, that the first
    great Aurora after autumn is well advanced, and following a period of fine
    weather, is a sign of a great storm of rain and wind in the forenoon of the
    second day afterwards.</p>

    <p>Mr. C. L. Prince, in his ‘Climate of Uckfield,’ p. 218, remarks that displays
    of Auroræ are almost invariably followed by very stormy weather, after an
    interval of from 10 to 14 days.</p>

    <h4 id="chap-6-3"><i>Aurora and the Magnetic Needle.</i></h4>

    <div class="sidenote">Aurora and
    the magnetic
    needle. Sir John
    Franklin’s
    observations.
    Motion communicated
    to the needle
    was neither
    sudden nor
    vibratory.
    Return of
    needle to its
    former position
    very
    gradual. Different
    positions of
    the Aurora
    had considerable
    influence
    on
    the direction
    of the
    needle. Needle disturbed
    when
    Aurora not
    visible. Quiescent
    yellow
    Aurora
    produced no
    perceptible
    effect on
    needle. Return of
    needle more
    speedy after
    formation
    of a second
    arch. Slow when
    disturbance
    was considerable.</div>

    <p>Sir John Franklin, in his ‘Narrative’ (before referred to), gives Lieutenant
    Robert Hood, R.N., the credit of being “the first who satisfactorily proved, by
    his observations at Cumberland House (before mentioned), the important fact
    of the action of the Aurora upon the compass-needle,” and also “to have
    proved the Aurora to be an electrical phenomenon, or at least that it induces
    a certain unusual state of electricity in the atmosphere.” Sir John Franklin
    then mentions that the motion communicated to the needle was neither sudden
    nor vibratory. Sometimes it was simultaneous with the formation of arches,
    prolongation of beams, or certain other changes of form or of activity of the
    Aurora. But generally the effect of these phenomena upon the needle was
    not visible immediately; but in about half an hour or an hour the needle had
    obtained its maximum of deviation. From this its return to its former position
    was very gradual, seldom regaining it before the following morning, and frequently
    not until the afternoon, unless it was expedited by another arch of
    the Aurora operating in a direction different from the former one. The magnetic
    needle in the open air was disturbed by the Aurora whenever it approached
    the zenith. Its motion was not vibratory (as observed by Mr. Dalton), perhaps
    owing to the weight of the card. It moved slowly to the E. or W. of the
    magnetic meridian, and seldom recovered its original direction in less than
    eight or nine hours. The greatest extent of its aberration was 45´. The
    arches of the Aurora were remarked commonly to traverse the sky nearly at
    right angles to the magnetic meridian; but deviation was not rare, and it was
    considered that the different positions of the Aurora had considerable influence
    on the direction of the needle. When an arch was nearly at right angles to
    the magnetic meridian, the motion of the needle was towards the W. This
    motion was greater when the extremity of the arch approached from the west
    towards the magnetic north. A westerly motion also took place when the
    extremity of an arch was in the true north, or about 36° to the west of the
    magnetic north. The motion of the needle was towards the east when the
    same end of an arch originated to the southward of the magnetic west, and
    when of course its opposite extremity approached nearer to the magnetic
    north. In one case only a complete arch was formed in the magnetic meridian.
    In another the beam shot up from the magnetic north to the zenith. In both
    these cases the needle moved towards the west. The needle was most
    disturbed on February 13, 1821, at a time when an Aurora was distinctly
    seen passing between a stratum of clouds and the earth. Sometimes the
    needle deviated though no Aurora was visible; but it was uncertain whether
    there might not have been a concealed Aurora at the time. Clouds were
    sometimes observed during the day to assume the form of the Aurora, and
    deviations of the needle were occasionally remarked at such times. An
    Aurora sometimes approached the zenith without producing any change of
    position of the needle; while at other times a considerable alteration took
    place, though the beams or arches did not come near the zenith. The Aurora
    was frequently seen without producing a perceptible effect on the needle. At
    such times it was generally an arch or a horizontal stream of dense yellowish
    light with little or no internal motion. The disturbance of the needle was
    not always proportionate to the agitation of the Aurora, but was always
    greater when the quick motion and vivid light were observed to take place
    in a hazy atmosphere. In a few instances the needle commenced at the instant
    a beam started from the horizon upwards; and its return was according to
    circumstances. If an arch formed immediately afterwards, having its extremities
    placed on opposite sides of the magnetic north and south to the former one,
    the return of the needle was more speedy, and it generally went beyond the
    point from which it first started. When the disturbance was considerable, it
    seldom regained its usual position before 3 or 4 <span class="smcapuc">P.M.</span> on the following day.
    On one occasion only the needle had a quick vibratory motion (between
    343° 50´ and 344° 40´). The disturbance produced by the Aurora was so great
    that no accurate deductions as to diurnal variation could be made.</p>

    <div class="sidenote">Magnetic
    observations
    on board the
    ‘Tegetchoff.’</div>

    <p>Payer, in his ‘New Lands within the Arctic Circle’ (vol. i. pp. 327, 328),
    gives the result of the magnetic observations on board the Austrian ship
    ‘Tegetchoff’ in the years 1872-74, made by means of a magnetic theodolite,
    a dipping-needle, and three variation instruments. The extraordinary disturbances
    of the needle rendered the determination of exact mean values for the
    magnetic constants impossible. The following were the principal results of
    these observations:—</p>

    <div class="sidenote">Disturbances
    great.</div>

    <p>(1) The magnetic disturbances were of extraordinary magnitude and
    frequency.</p>

    <div class="sidenote">Greater
    as the rays
    were rapid. Quiescent
    arches exercised
    no
    influence.</div>

    <p>(2) They were closely connected with the Aurora, and they were greater as
    the motion of the rays was more rapid and fitful and the prismatic colours
    more intense. Quiescent and regular arches, without changing rays or
    streamers, exercised mostly no influence on the needle.</p>

    <div class="sidenote">Declination-needle,
    effects on.</div>

    <p>(3) In all the disturbances the declination-needle moved towards the east,
    and the horizontal intensity decreased while the inclination increased.</p>

    <p>Sir John Franklin sums up his information as to the needle to much
    the same effect, viz. that brilliant and active coruscations cause a deflection
    almost invariably if they appear through a hazy atmosphere and if
    the prismatic colours are exhibited in the beams or arches. On the contrary,
    when the air is clear and the Aurora presents a steady dense light of a
    yellow colour and without motion, the needle is often unaffected by its
    appearance.</p>

    <span class="sidenote">Parry’s experience.</span>

    <p>Parry (Third Voyage) found his variation-needle (extremely light and
    delicately suspended) in no instance affected by the Auroræ; but he seems to
    have principally met with the quiescent form of that phenomenon.</p>

    <p>M. Lottin, the French savant (whose description of an Auroral display has
    been given in Chapter II.), observed in the North Sea, between September
    1838 and April 1839, while the sun was below the horizon, 150 Auroræ.
    During this period 64 were visible, “besides many which a cloudy sky
    concealed, but the presence of which was indicated by the disturbances they
    produced upon the magnetic needle” (Lardner’s ‘Museum of Science and Art,’
    vol. x. p. 189).</p>

    <div class="sidenote">Grand displays
    accompanied
    by motion
    of needle to
    the west.</div>

    <p>It has been remarked by some observers that grand displays of the Aurora
    are frequently preceded or accompanied by an extraordinary motion of the
    needle to the westward.</p>

    <p>Captain Maguire found at Point Barrow (1852-54) that the appearance of
    the Aurora in the south was connected with the motion of the magnet to the
    east of the magnetic north, and if in the north to the west of the same.</p>

    <div class="sidenote">Solar disturbances
    and Aurora.</div>

    <p>On an occasion in 1859 great solar disturbances were observed, the
    Greenwich magnets were much disturbed, and a fine Aurora was visible.</p>

    <div class="sidenote">Cipoletti’s
    observation.</div>

    <p>Cipoletti, of Florence, remarks on the strong magnetic disturbances at
    Vienna and Munich during the Auroræ of 4th February, 1872, and 4th
    February, 1874.</p>

    <div class="sidenote">Dr. Thompson
    concludes
    that
    cylinders of
    Aurora
    cannot be
    doubted to
    be magnets.</div>

    <p>Dr. Thompson, in his ‘Annals of Philosophy,’ vol. iv. p. 431 (1814), mentions
    as an authenticated fact that during the prevalence of the Aurora the magnetic
    needle was frequently observed to become unsteady, and (p. 432) concludes
    that cylinders of Aurora cannot be doubted to be magnets. The only three
    bodies capable of assuming magnetic properties are iron, nickel, and cobalt.
    When meteors are considered, it is not altogether extravagant to conjecture
    that bodies similar in their nature to some of the solid bodies which constitute
    our globe may exist in some unknown state in the atmosphere.</p>

    <p>During the Aurora of 13th May, 1869, the declination at Greenwich varied
    1° 25´, while the vertical force experienced four successive maxima, and
    the greatest oscillation amounted to 0·04 of the total mean value. The horizontal
    force varied only 0·014 of its mean value.</p>

    <p>During the Aurora of 15th April, 1869, the declination at Stonyhurst varied
    2° 23´ 14″ in nine minutes.</p>

    <figure class="plate" id="plate9">
      <!-- <img src="assets/aurorae/images/plate9.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-6-4"><i>Auroræ, Magnetic Disturbances, and Sun-spots.</i></h4>

    <div class="sidenote">Auroræ,
    magnetic
    disturbances,
    and
    sun-spots
    in Italy.</div>

    <p>Auroræ were frequent in Italy in April 1871. On the 10th a remarkable
    one was seen, with declinometer deflected towards the east, and 63 sun-spots
    were counted. On the morning of the 10th the deflection continued, and at
    midday 97 sun-spots were counted.</p>

    <p>On the 18th a brilliant Aurora lasted to 10 o’clock at night. From this
    time till the 23rd the Aurora appeared constantly, giving a reddish tinge
    in the north and north-west. A brilliant display took place on the evening
    of the 23rd. On the evenings when the Aurora appeared the magnetometers
    were disturbed throughout Italy, and ended by a violent agitation
    during the whole of the 24th. Sun-spots were observed at Rome, Palermo,
    and Moncalieri, but the greater number on the days of the Auroræ. A
    brilliant display at Moncalieri on June 18 was accompanied by very violent
    magnetic disturbance.</p>

    <div class="sidenote">Proctor’s
    sun-spots
    and Aurora.</div>

    <p>September 25, 1870, Mr. Proctor counted 102 spots on the solar disk; and
    on the night of the 24th and morning of the 25th an Aurora of unwonted
    magnificence was visible at various stations in England, France, and Germany.</p>

    <div class="sidenote">Sun-spots
    and the
    magnet.
    11 years’
    period.
    Schwabe’s
    sun-spot
    period.</div>

    <p>With respect to sun-spots and the magnet, the frequency of magnetic
    storms, causing oscillation of the needle, gradually increased from a minimum
    in 1843 to a maximum in 1848, giving a variation of something near 11
    years altogether. Schwabe observed the sun-spots for 24 years, and found
    they had a regular maximum and minimum every five years, and that the
    years 1843 and 1848 were minimum and maximum years coinciding with the
    magnetic variation at those periods.</p>

    <div class="sidenote">Prof.
    Loomis
    considers
    connexion
    established
    between
    magnetic
    declination,
    auroral displays,
    and
    sun-spots.</div>

    <p>Professor Loomis (‘American Journal of Science,’ vol. v. April 1873)
    considers that a comparison between the mean daily range of the magnetic
    declination and the number of Auroras observed in each year, and also with
    the extent of the black spots on the surface of the sun, establishes a connexion
    between these phenomena, and indicates that auroral displays (at
    least in the middle latitudes of Europe and America) are subject to a law
    of periodicity, that their grandest displays are repeated at intervals of
    about 60 years, and that there are also other fluctuations, less distinctly
    marked, which succeed each other at an average interval of about 10 or 11
    years, the times of maxima corresponding quite remarkably with the maxima
    of solar spots.</p>

    <div class="sidenote">Illustrative
    table of coincidences.</div>

    <p>An illustration of the result of these observations is given on Plate IX.
    fig. 2. The curves are in close correspondence, and the coincidence at the
    times of maximum and minimum is remarkable. The auroral maximum
    generally occurs a little later than the magnetic maximum; and the connexion
    between the auroral and magnetic curves appears somewhat more
    intimate than between the auroral and sun-spot curves.</p>

    <div class="sidenote">Prof.
    Loomis
    considers
    a sun-spot
    a solar disturbance
    affecting the
    earth’s magnetism.</div>

    <p>Professor Loomis contends “that the black spot is a result of a disturbance
    of the sun’s surface, which is accompanied by an emanation of some
    influence from the sun, which is almost instantly felt upon the earth in an
    unusual disturbance of the earth’s magnetism, and a flow of electricity, developing
    the auroral light in the upper regions of the earth’s atmosphere.”</p>

    <div class="sidenote">Carrington
    and Hodgson’s
    observations
    of
    bright spots
    on the sun,
    accompanied
    by
    magnetic
    disturbance
    at Kew, and
    followed by
    wide-spread
    Auroræ.</div>

    <p>This connexion between the sun’s spots and the earth’s magnetism has
    been considered as proved; and one instance at least of an intense disturbance
    and outbreak of the sun’s surface having been observed simultaneously with
    the occurrence of a terrestrial magnetic storm is a matter of record. This
    will be found detailed in the ‘Monthly Notices of the Royal Astronomical
    Society,’ vol. xx. pp. 13 and 15, and is so interesting in its character that it
    may be briefly referred to here. Mr. R. C. Carrington, September 1, 1859,
    11h 18m, while observing and drawing a group of solar spots, saw suddenly
    two patches of intense bright light break out in the middle of the group.
    The brilliancy was fully equal to that of direct sunlight. Seeing the outbreak
    was on the increase, Mr. Carrington left the telescope, to call some one to
    witness it. On his return within sixty seconds it was nearly concluded. The
    spots travelled from their first position, and vanished as two rapidly fading
    dots of white light. In five minutes the two spots traversed a space of about
    35,000 miles. Mr. Carrington found no change in the group itself. His
    impression was that the phenomena took place at an elevation considerably
    above the general surface of the sun, and above and over the great group of
    spots on which it was seen projected. It broke out at 11h 18m, and
    vanished at 11h 23m. Mr. R. Hodgson independently on the same day, and at
    close upon the same time, saw a very brilliant star of light, much brighter
    than the sun’s surface, most dazzling to the protected eye, illuminating the
    upper edges of the adjacent spots and streaks. The rays extended in all
    directions, and the centre might be compared to α Lyræ when seen in a large
    telescope. It lasted for some five minutes.</p>

    <p>At the very moment of this solar disturbance the instruments at Kew indicated
    a <i>magnetic storm</i>; and Proctor, in his volume on the Sun, page 206,
    details how this magnetic storm was accompanied by very widely-spread
    indications of electrical disturbance in many parts of the globe. Vivid
    Auroræ were seen not only in both hemispheres, but in latitudes and places
    where they are seldom witnessed. Rome, Cuba, and the West Indies, the
    tropics within 18° of the equator, and even South America and Australia, are
    thus referred to for displays. At Melbourne, on the night of September 2nd,
    the greatest Aurora ever seen there made its appearance.</p>

    <p>It was observed, too, that magnetic communication was at the same time
    disturbed all over the earth. Strong currents, continually changing their
    direction, swept along the telegraphic wires. At Washington and Philadelphia
    the signal-clerks received severe shocks, and the wires had to give up
    work. At a station in Norway the transmitting apparatus was set fire to;
    and at Boston, in North America, a flame of fire followed the pen of Baine’s
    electric telegraph.</p>

    <div class="sidenote">Mr. John
    Allan
    Broun’s
    magnetic
    oscillation-curves;
    showing
    that the sun’s magnetic
    action
    has lately
    become more
    constant. In diagram,
    curves gradually
    flatten.</div>

    <p>In an interesting communication to ‘Nature’ (January 3rd, 1878), entitled
    “The Sun’s Magnetic Action at the Present Time,” Mr. John Allan Broun
    has contributed some magnetic oscillation-curves, deduced from observations
    made in the Trevandrum Observatory (nearly on the magnetic equator),
    by which, if confirmed by other observations, it would appear that the sun’s
    magnetic action has lately become gradually more constant. The curves are
    three in number,—no. 1 for the years 1855-58, no. 2 for the years 1865-68,
    no. 3 for the years 1874-77. In no. 1 curve the minimum is very clearly
    marked by two points corresponding to April 1 and May 1, 1856, and there
    is little difference in the rapidity with which the curve descends to and
    ascends from the minimum. In no. 2 curve the epoch of minimum is by no
    means so well marked; it occurs between the points for April 1 and September
    1, 1866. There is also a considerable difference in the rapidity of
    variation in the descending and ascending branches of the curve. The
    descent is nearly as rapid as in curve no. 1; but the ascent is very much
    slower. In curve no. 3 the lowest point is that for December 1, 1875; but
    it is even now, with points a year and a half later, difficult to say whether
    this is the minimum or not, the point for January 1, 1877, being only 0·02
    (two hundredths of a minute of arc) higher. In this curve the change of
    range in diurnal oscillation is quite insignificant from November 1, 1874, to
    April 1, 1877, an interval of three years and five months. In the diagram
    given by Mr. Broun the curves show themselves gradually flattening, no. 3
    being almost a straight line.</p>

    <div class="sidenote">Mr. Broun
    never found
    an Aurora
    without a
    corresponding
    irregularity
    in the
    declination-needle.</div>

    <p>Mr. Broun remarks upon the report of Sir George Nares as to the insignificant
    nature of the Auroræ seen in the Arctic Expedition in the winter of
    1875-76, and the accompanying statement that, as far could be discovered,
    they were totally unconnected with any magnetic or electric disturbance; and
    states, as the result of his own experience in the south of Scotland, that several
    of the Auroræ observed by him were of the very faintest kind, “were traces”
    which he could never have remarked had he not been warned by very slight
    magnetic irregularities to examine the sky with the greatest attention.
    Again, in no case had he seen the faintest trace of an Aurora without finding
    at the same time a corresponding irregularity in the movement of the force or
    declination-magnet.</p>

    <div class="sidenote">Prof. Piazzi
    Smyth comments
    on
    variance in
    the cycles.</div>

    <p>Prof. Piazzi Smyth, commenting on this article, makes the inquiry how the
    sun-spot cycle and the terrestrial magnetic oscillation cycle can be considered
    as agreeing, the sun-spot cycle, according to Prof. Wolf, being 11·111 years,
    and the magnetic cycle 10·5 years according to Mr. Broun.</p>

    <div class="sidenote">M. Faye’s
    remarks to
    a similar
    effect.</div>

    <p>Another correspondent writes and quotes M. Faye, in ‘La Météorologie
    Cosmique,’ for the remark, “La période des taches portée à 11 ans ·1 par M.
    Wolf n’étant pas égale à celle des variations magnétiques (10 ans ·45), ces
    deux phénomènes n’ont aucun rapport entre eux.”</p>

    <div class="sidenote">Mr. Broun’s
    rejoinder
    and explanation.</div>

    <p>Mr. Broun, in a further letter, rejoins that if we could accept Dr. Wolf’s
    view we should find that the mean duration of a cycle for <i>both</i> phenomena
    since 1787 would be 11·94 years, while the sun-spot results for eight cycles
    determined by Dr. Wolf during eighty years before 1787 give 10·23 or, if we
    take nine cycles, 10·43 years for the mean duration. It is by mixing these
    two very different means that the Zurich philosopher finds 11·1 years, a mean
    which Mr. Broun considers can evidently have no weight given to it. On
    the other hand, if Dr. Wolf is in error (as Mr. Broun believes he is) as to
    the existence of a maximum in 1797, the mean durations for the eighty years
    after and for the eighty years before 1787 agree as nearly as the accuracy of
    the determinations for the beginning of the eighteenth century will permit.
    Mr. Broun then repeats his conviction that the sun-spot maxima and minima
    are really synchronous with those of the magnetic diurnal observations.</p>

    <div class="sidenote">Mr. Jenkins’s
    explanation
    of
    Prof.
    Loomis’s
    chart.</div>

    <p>Mr. B. G. Jenkins, in a letter to ‘Nature,’ refers Prof. Smyth to Prof.
    Loomis’s chart of magnetic oscillations given in Prof. Balfour Stewart’s paper
    in ‘Nature’ (vol. xvi. p. 10), for the purpose of showing that there are exactly
    seven minimum periods from 1787 to 1871, the mean of which is twelve
    years, the mean of the seven corresponding maximum periods being 11·8
    years. The true magnetic declination-period is, then, the mean of these, viz.
    11·9 years. In exactly the same manner he finds that the mean period
    of sun-spots is 11·9 years.</p>

    <div class="sidenote">Jupiter’s
    suspected
    connexion
    with sun-spots.</div>

    <p>The auroral displays also have the same period. Mr. Jenkins also refers
    to Wolf, De La Rue, Stewart, and Loewy, as having stated their belief that
    Jupiter is the chief cause in the production of sun-spots, and draws attention
    to the period of 11·9 years as being Jupiter’s anomalistic year, or the time
    which elapses between two perihelion passages.</p>

    <div class="sidenote">Infrequency
    of
    Auroræ and
    absence of
    sun-spots in
    1876-78.</div>

    <p>The infrequency of Auroræ during the years 1876-78, and a corresponding
    comparative absence of sun-spots, may be added to the evidence on the
    subject. I have seen no account of important Auroræ during the years
    mentioned, and day after day has recently (1878) passed with a perfectly
    clean sun-disk.</p>

    <h4 id="chap-6-5"><i>Aurora and Electricity.</i></h4>

    <div class="sidenote">Aurora and
    electricity. Sir John
    Franklin’s
    experience
    with electrometer.</div>

    <p>Sir John Franklin failed to get indications of electricity connected with the
    Aurora with a pith-ball electrometer; but with another form of electrometer
    specially constructed for the purpose he seems to have got some, though not
    very strong or regular, indications of repulsion between the needle of the
    instrument and the conductor when Auroræ were seen. He does not decide
    whether the electricity was received from or summoned into action by the
    Aurora.</p>

    <div class="sidenote">Parry’s
    experience.</div>

    <p>Parry, at Fort Bowen, with a gold leaf electroscope connected with a chain
    attached by glass rods to the skysail mast-head, 115 feet above sea-level, found
    no effect.</p>

    <div class="sidenote">Dr. Allnatt’s
    experience,
    February 4,
    1872.</div>

    <p>Dr. Allnatt, at Frant, during the display of 4th February, 1872, found the
    earth’s electricity so powerful that the gold leaves of the electrometer remained
    divergent for a considerable time.</p>

    <div class="sidenote">M’Clintock
    found electroscope
    affected in
    Baffin’s Bay
    and Port
    Kennedy.</div>

    <p>M’Clintock observes that on six occasions of Aurora in Baffin’s Bay, the
    electroscope was strongly affected, and on three occasions of Aurora at Port
    Kennedy. The electricity was always positive.</p>

    <p>Dec. 18.—Dr. Walker called him to see the electroscope. The charge was
    at first weak, but afterwards strong enough to keep the leaves diverged.
    Dr. Walker found two periods of minimum electrical disturbance about 9 <span class="smcapuc">P.M.</span>
    and noon.</p>

    <div class="sidenote">Electric
    currents in
    telegraphic
    wires during
    Auroræ.</div>

    <p>Electric currents have been reported as produced in telegraph wires during
    Auroræ. Though transient they are said to be often very powerful, and to
    interrupt the ordinary signals. Loomis (Sillim. Journ. vol. xxxii.) mentions
    cases where wires have been ignited, brilliant flashes produced, and combustible
    materials kindled by their discharge.</p>

    <p>Here, too, we may note the account of electric phenomena in the case of the
    Aurora Australis described (<i>antè</i>, p. 28) by Mr. Proctor.</p>

    <div class="sidenote">Mr. George
    Draper’s
    report as to
    disturbed
    condition of
    the Indian
    Submarine
    and other
    cables
    during
    Aurora of
    February 4,
    1872.</div>

    <p>Mr. George Draper, of the British-Indian Submarine Telegraph Company,
    speaking of the Aurora of February 4, 1872 (and writing to the ‘Times’
    under date February 5th), states that the Aurora visible in London was also
    visible at Bombay, Suez, and Malta, and that the Company’s electrician at
    Suez reported that the earth-currents there were equal to 170 cells (Daniell’s
    battery), and that sparks came from the cable. The electrical disturbances
    lasted until midnight, and interrupted the working of both sections of the
    British Indian Cable between Suez and Aden, and Aden and Bombay. For
    some days previously the signals on the British Indian cables had been much
    interfered with by electrical and atmospheric disturbances.</p>

    <p>At Malta there was a severe storm on the morning of the 4th, so that it
    was necessary to join the cable to earth for some hours, and the Aurora was
    very large and brilliant there.</p>

    <p>The electrical disturbances on the cables in the Mediterranean and on those
    between Lisbon and Gibraltar, and Gibraltar and the Guadiana, were also
    very great. The signals on the land line between London and the Land’s End
    were interrupted for several hours on the night of the 4th by atmospheric
    currents. Similar effects accompanied the displays of Oct. 24 and 25, 1870.</p>

    <h4 id="chap-6-6"><i>Aurora and Meteoric Dust.</i></h4>

    <div class="sidenote">Aurora and
    meteoric
    dust. Theories of
    Dr. Zeyfuss
    and M.
    Gröneman.</div>

    <p>A theory has been propounded independently by Dr. Zeyfuss and by
    M. Gröneman, of Gröningen, according to which the light of the Aurora is
    caused by clouds of ferruginous meteoric dust ignited by friction with the
    atmosphere. Gröneman shows that these might be arranged along the magnetic
    curves by the action of the earth’s magnetic force during their descent,
    and that their influence might produce the observed magnetic disturbances.</p>

    <div class="sidenote">Ferruginous
    dust in the
    Polar
    Regions.</div>

    <p>The arches might be accounted for by the effects of perspective; and the
    iron spectrum shows correspondence with some of the lines of the Aurora.
    Ferruginous particles have been found in the dust of the Polar regions according
    to Professor Nordenskiöld, but whether derived from stellar space or from
    volcanic eruption is uncertain. A difficulty has been suggested that while
    meteors are more frequent in the morning, or on the face of the earth which
    is directed forward on its orbit, the reverse prevails in the case of Auroræ.
    Gröneman meets this by supposing that in the first case the velocity may be
    too great to allow of arrangement by the earth’s magnetic force. He accounts
    for the infrequency of the Aurora in equatorial regions by the weakness of the
    earth’s magnetic force, and the fact that when it does occur the columns must
    be parallel to the earth’s surface.</p>

    <div class="sidenote">Baumhauer’s
    proposition.</div>

    <p>Baumhauer (Compt. Rend. vol. lxxiv. p. 678) advances, as regards Polar
    Auroræ, the proposition, that not only solid masses large and small, but clouds
    of “uncondensed” (meteoric) matter probably enter our atmosphere.</p>

    <p>If from our knowledge of the meteoric stones which fall to the earth’s
    surface we may draw any conclusion respecting the chemical constitution of
    these clouds of matter, it would appear that they may contain a considerable
    portion of the magnetic minerals iron and nickel. Let such a cloud approach
    our earth, regarded as a great magnet, it would be attracted towards the
    Pole, and, penetrating our atmosphere, the particles which have not been
    oxidized, and are in a state of extremely fine division, would by their oxidation
    generate light and heat, producing the polar Auroræ. Baumhauer suggests
    it would be interesting in support of this theory to detect in the soil of polar
    areas the presence of nickel. The presence of iron and nickel in meteoric
    masses in considerable quantities is frequent; and cases are also on record by
    Eversmann of hailstones containing crystals of a compound of iron and sulphur,
    by Pictet of hailstones containing nuclei which proved to be iron, and by
    Cozari of hailstones containing nuclei of an ashy-grey colour, the larger ones
    of which were attracted by the magnet, and found to contain iron and nickel.
    Nickel was found by Reichenbach in parts of Austria on hills consisting of
    beds of sandstone and limestone, and quite free from metallic veins.</p>

    <div class="sidenote">Mr. Lefroy’s
    description
    of a phenomenon
    ascribed by
    him to
    streams of
    cosmic dust.</div>

    <p>Mr. J. W. N. Lefroy, in ‘Nature,’ describes a phenomenon seen by him at
    Fremantle, West Australia, in the month of May, which he designates
    “A Lunar Rainbow, or an Intra-lunar convergence of Streams of slightly
    illuminated Cosmic Dust?”</p>

    <p>It lasted about three quarters of an hour, and consisted of one grand central
    feather, of very bright white cloud, springing out of the horizon at W.N.W.,
    and crossing the meridian at about 20° north of the zenith, with a width of
    7° to 8°.</p>

    <p>On either side of this was a system of seven or eight minor beams of light,
    extending from the W. to the E. horizon, subtending a chord common to themselves
    and to the main stream, and converging towards the axis of the central
    stream so as to intersect it at a point about 30° or 40° below the western horizon,
    at which the whole system subtended an azimuth of about 20°. Near the zenith,
    where its transverse section was a maximum, that section subtended an angle
    of about 40°.</p>

    <p>The idea strongly suggested itself to Mr. Lefroy of converging streams of
    infinitely minute particles of matter passing through space at a distance from
    the earth at which its aerial envelope may have still a density sufficient by
    its resistance to give cosmic dust passing through it that illumination which
    it possessed. In about twenty minutes the streams of light had attained their
    maximum brightness. Their apparent figure was that of a nearly circular
    (slightly flattened) arc of an amplitude of 15° or 20°, as viewed from the middle
    point of its chord.</p>

    <p>The brightness and the convergence of the streams were both more marked
    towards the western horizon than the eastern. This same phenomenon was
    described in the ‘South-Australian Register’ as a beautiful lunar rainbow
    visible in the western heavens.</p>

    <p>Mr. Lefroy and other observers concurred in the impression that the minor
    lateral streams on the N. side of the main one intervened between the earth
    and the moon, and that one or more of them in their slow vibrations swept the
    surface of the moon and sensibly obscured its light. There can be hardly any
    question that the phenomenon observed was in fact an Aurora.</p>

    <div class="sidenote">Suggestion
    as to collecting iron and
    nickel particles
    from the
    atmosphere.</div>

    <p>It may be a question whether iron and nickel particles of meteoric origin
    do not ordinarily exist in the atmosphere in a greater degree than we suspect,
    and might be detected if special means, such as magnets, plates of glass
    covered with glycerine, &amp;c., were adopted for the purpose of collecting and
    examining the cosmic dust. Larger gatherings than usual of iron and nickel
    particles during the presence of Auroræ would be in support of Mr. Lefroy’s
    theory.</p>

    <h4 id="chap-6-7"><i>The Aurora and the Planets Venus and Jupiter.</i></h4>

    <div class="sidenote">The Aurora
    and planets
    Venus and
    Jupiter. The planet
    Venus’s halo
    during
    Aurora.</div>

    <p>During a brilliant Aurora seen at Sunderland, February 8, 1817 (‘Annals
    of Philosophy,’ p. 250), about 8 o’clock, Venus was about 8° above the horizon,
    and displayed a very peculiar appearance. Her rays passed through a thin
    mist or cloud, probably electric, of a deep yellow tint. Her apparent magnitude
    seemed increased, and a halo was formed round her as sometimes
    appears round the moon in moist weather; but the stars that were in that
    part of the heavens shone with their accustomed brilliancy.</p>

    <div class="sidenote">Dr. Miles’s
    observation
    of Venus
    during an
    Aurora.</div>

    <p>The Rev. T. W. Webb, in his ‘Celestial Objects’ (1859), p. 43, quoting
    from the Philosophical Transactions, mentions that, “January 23rd, 1749-50,
    there was a splendid Aurora Borealis about 6 <span class="smcapuc">P.M.</span> The Rev. Dr. Miles, at
    Tooting, had been showing Jupiter and Venus to some friends with one of
    Short’s reflectors, greatest power 200, when a small red cloud of the Aurora
    appeared, rising up from the S.W. (as one of a deeper red had done before),
    which proceeded in a line with the planets and soon surrounded both. Venus
    appearing still in full lustre, he viewed her again with the telescope without
    altering the focus, and saw her much more distinctly than ever he had done
    upon any occasion. His friends were of the same opinion. They all saw her
    spots plain (resembling those in the moon), which he had never seen before,
    and this while the cloud seemed to surround it as much as ever.”</p>

    <p>I think this effect might perhaps have arisen from the Aurora acting as a
    screen, and removing the glare with which so bright an object as Venus is
    always accompanied; but the case is a singular one, and one would be glad of
    further experience. I suggested observations on this head during Sir Geo.
    Nares’s Arctic Expedition; but the suggestion, for some reason of which I am
    not aware, was not included in the official instructions issued.</p>

    <div class="sidenote">Brightness
    of stars
    during
    Auroræ.</div>

    <p>Remarks are frequent of the brightness of stars as seen through Auroræ.
    Payer, of the Austrian Expedition, remarks that falling stars passed through
    the Aurora without producing any perceptible effect or undergoing any change.</p>

    <div class="sidenote">Aurora of
    Oct. 24,
    1870, and
    Jupiter.</div>

    <p>A grand display of the Aurora took place 24th October, 1870. About this
    time the belts of Jupiter were observed to be highly coloured. As observed
    by me on the night of November 2, 1870, at 9 <span class="smcapuc">P.M.</span>, with an 8¼-in. Browning
    reflector, achromatic eyepieces 144, 305, and 450, the equatorial zone was of
    a distinctly dark ochre colour, deepening to red-brown as it approached the
    lower (N.) edge. Two thin belts above were slate-purple, and a darker belt
    below was of a deep purple colour.</p>

    <div class="sidenote">According
    to Lassell
    and others,
    Jupiter’s
    belts exhibit
    the brightest
    colours at
    period of
    Auroræ.</div>

    <p>Lassell, Proctor, and others have reported Jupiter’s belts to exhibit the
    brightest colours at the period of Auroræ. Mr. Browning gives a drawing of
    Jupiter as seen on January 31, 1870 (a year noted for Auroræ), with the belts
    brightly coloured. The finest view of Jupiter I ever had was on the
    8th February, 1872 (a fine Aurora was on the 4th), when, with the 8¼-inch
    Browning reflector, I saw the whole surface of the planet (by glimpses) cloud-mottled.
    The equatorial belt was, however, then slightly tinted only. In
    Dr. Miles’s observation (p. 66) he does not seem to have noticed the colouring
    of Jupiter’s belts.</p>

    <div class="sidenote">Infrequency
    of Auroræ
    and lightness
    in tint
    of Jupiter’s
    belts.</div>

    <p>The three past years, 1876, 1877, and 1878, have been distinguished by the
    infrequency of Auroræ; and Jupiter’s equatorial zone and belts have been
    mainly reported of light tints.</p>

    <p>The subject apparently deserves more attention than it has hitherto received.</p>

    <h4 id="chap-6-8"><i>The Aurora and the Zodiacal Light.</i></h4>

    <div class="sidenote">The Aurora
    and the
    Zodiacal
    Light. Ångström’s
    observation
    on spectrum.</div>

    <p>Ångström in 1867 found the spectrum of the Zodiacal Light to be monochromatic,
    consisting of a single line in the green, to which he assigned
    approximately the position 1259 on Kirchhoff’s scale, the same that he had
    determined for the green line of the Aurora Borealis; and Respighi, on the
    Red Sea, on the evening of the 11th and the morning of the 12th January
    1872, perceived in the Zodiacal Light not only this green line, but near it,
    towards the blue, a band or zone of apparently continuous spectrum.</p>

    <div class="sidenote">Respighi’s
    at Campidoglio.</div>

    <p>At the Observatory of the Royal University of Campidoglio, February 5th,
    1872, Respighi, at 7 <span class="smcapuc">P.M.</span>, was able to discern the same spectrum; and on
    directing the spectroscope to other points he found that this spectrum showed
    itself in all parts of the heavens from the horizon to the zenith, more or less
    defined in different parts, but everywhere as bright as in the Zodiacal Light.
    The Observatory Assistant, Dr. di Legge, likewise observed this spectrum
    distinctly in various parts of the heavens. Respighi’s observations corroborating
    Ångström’s in 1867, appeared to him to demonstrate the identity
    of the Zodiacal Light with the Aurora, and to establish the identity of their
    origin.</p>

    <div class="sidenote">Pringle
    thinks the
    Aurora may
    be considered
    as
    allied to the
    Zodiacal
    Light.</div>

    <p>Pringle, in a letter to ‘Nature’ from South Canara, October 3, 1871,
    alludes to the Aurora as being considered by many allied to the Zodiacal
    Light, and does not think the evidence then hitherto adduced against the
    theory at all conclusive. He says:—“Assume the auroral light to consist of
    solid particles of matter, planet dust, shining by reflected light, and it is not
    difficult to imagine the Aurora playing amongst these tiny worlds, each of
    which would have its own small magnetic system swayed like our own by the
    monster magnet the sun.”</p>

    <div class="sidenote">Phosphorescence
    of sky
    when Zodiacal
    Light
    has been
    seen bright.</div>

    <p>He notices he has never found it to have a decided outline, nor traced it
    east or west to 180° from the sun. He also refers to others having noticed that
    when the Zodiacal Light has been seen unusually bright, a “phosphorescence”
    of the sky was everywhere visible.</p>

    <div class="sidenote">Pringle
    failed to
    find bright
    lines or
    bands in the
    Zodiacal
    Light.</div>

    <p>He does not seem at that time to have examined the matter spectroscopically;
    and on June 23, 1872, he writes again, pointing out the peculiarity
    in Respighi’s observation that the green line was seen everywhere as bright
    as in the Zodiacal Light, and suggesting that it was due to a concealed Aurora
    present at the time of Ångström’s and Respighi’s observations. He further
    states he had examined the Zodiacal Light with a Browning 5-prism spectroscope
    (I presume a compound direct-vision form is meant) since the last
    December, and, brilliant as the phenomenon had frequently been, failed to
    detect the slightest appearance of bright lines or bands. A faint diffuse
    spectrum about as intense as that of a bright portion of the Milky Way was
    all he had obtained.</p>

    <div class="sidenote">Prof. Piazzi
    Smyth
    confirms
    this.</div>

    <p>Professor Piazzi Smyth, in the clear sky of Italy, and with an instrument
    specially designed for showing faint spectra, found no lines or bands, but only
    a faint continuous spectrum extending from about midway between D and E
    in the solar spectrum to nearly F (see Plate V. fig. 3, in which the continuous
    spectrum is graphically shown, white on a black ground).</p>

    <div class="sidenote">Colour of
    the Zodiacal
    Light.</div>

    <p>It may here be mentioned that the Zodiacal Light is usually described as,
    in these latitudes, of a golden yellow or pale lemon tinge.</p>

    <div class="sidenote">Rev. Mr.
    Webb’s observation,
    February 2,
    1862. He found no
    green line of
    the Aurora.</div>

    <p>On one occasion, however, it has been described as not having this tinge,
    but rather resembling the light of the Milky Way, but brighter. On another
    occasion I saw the whole cone of a crimson hue without any mixture of yellow.
    The Rev. Mr. Webb thought that a display seen at Hardwick Vicarage,
    February 2nd, 1862, showed a ruddy tinge not unlike the commencement of
    a crimson Aurora—“it was certainly redder or yellower than the galaxy.” He
    examined it with a pocket spectroscope which would show distinctly the green
    line of the Aurora (probably Browning’s miniature), but nothing of the kind
    was visible, nor could any thing be traced beyond a slight increase of general
    light, which, on closing the slit, was extinguished long before the auroral band
    would have become imperceptible.</p>

    <div class="sidenote">A. W.
    Wright’s
    observations
    and conclusions.</div>

    <p>A. W. Wright examined the Zodiacal Light with a Duboscq single-prism
    spectroscope, the telescope and collimator having a clear aperture of 2·4 centimetres,
    magnifying-power of telescope 9 diameters. Special precautions
    were taken about the observations, and the conclusions arrived at were:—</p>

    <p>(1) The spectrum of the Zodiacal Light is continuous, and is sensibly the
    same as that of faint sunlight or twilight.</p>

    <p>(2) No bright line or band can be recognized as belonging to this spectrum.</p>

    <p>(3) There is no evidence of any connexion between the Zodiacal Light and
    the Polar Aurora.</p>

    <div class="sidenote">Polarization
    of Zodiacal
    Light. Burton’s
    observation
    confirmed
    by Wright
    and
    Tacchini.</div>

    <p>The Polarization of the Zodiacal Light has been already referred to under
    the head of “Polarization of the Aurora:” but it may be here noted that
    Mr. Burton’s observation of polarization of the light there mentioned has
    been confirmed by Wright and Tacchini, and the presence of reflected sunlight
    established. In this respect it differs from the Aurora, in which no
    trace of polarization has hitherto been detected; and looking at this, and at
    the weight of evidence in the spectroscopic observations, the theory of a connexion
    between the Aurora and the Zodiacal Light must, as the matter stands,
    be given up.</p>

  </section>

  <!-- Chapitre 7_________________________________________________________-->
  <section class="chapter" id="chapter-7">
    <h3 class="titlechapter" id="chap-7">Aurora-like patches on the partially-eclipsed Moon</h3>
    <p class="shorter">Aurora-like patches on the partially-eclipsed Moon</p>

    <div class="sidenote">Aurora-like
    patches on
    the partially-eclipsed
    moon,
    Feb. 27,
    1877.</div>

    <p>In anticipation of the total eclipse of the Moon on the 27th February, 1877,
    several articles appeared in the leading journals of the day describing, for the
    public benefit, the appearances which might be expected during the occurrence
    of the phenomenon.</p>

    <div class="sidenote">Formerly it
    was thought
    the moon
    was illuminated
    by
    auroral light.</div>

    <p>Among these was one by Mr. R. A. Proctor, in which the following passage
    occurs:—“That dull, or occasionally glowing red colour, shown by the moon
    when she is fully and even deeply immersed in the shadow of the earth, is a
    phenomenon whose explanation is not without interest. Formerly it was
    thought that the moon possessed an inherent light, or <i>perhaps was illuminated
    by auroral light</i>, which only became discernible at the time of total eclipse.
    Indeed even Sir W. Herschel fell into the mistake of supposing this the only
    available explanation, having miscalculated the efficiency of the true cause.”</p>

    <div class="sidenote">Author’s
    notes of the
    eclipse. Colour-tints
    described. A crimson-scarlet
    tint
    reminded
    author of an
    auroral
    glow.</div>

    <p>This passage was only pointed out to me by a friend after the eclipse had
    actually taken place, and I had sent him some notes of what I then saw. My
    notes on the occasion comprised, amongst others, the following remarks:—“The
    tints of colour also during partial eclipse, owing, no doubt, to the
    moon’s considerable altitude, were singularly bright and well contrasted.
    Silver-grey, dusky copper-red, and the same tint clearer and brighter were
    ranged side by side with a lovely jewel effect. <i>We noticed also at times a
    crimson-scarlet tint, deeper and less mixed with yellow than the copper colour.</i>
    This last tint reminded me much of a <i>crimson glow common to the Aurora</i>,
    and which I also once distinctly remarked (of course in a weaker degree) in
    the zodiacal light” (<i>antè</i>, p. 68).</p>

    <div class="sidenote">Eclipse, Aug.
    23-24, 1877. Sky clear,
    but eclipsed
    moon misty
    and indistinct
    until
    total obscuration. Succession
    of colours.</div>

    <p>On the occasion of the eclipse of August 23-24, 1877, we were favoured
    at Guildown, in common with many other places, by a singularly clear sky
    during the progress of the moon’s obscuration and subsequent clearing. In
    the early part of the evening, however, the moon, from some cause (possibly
    atmospheric vapour), seemed to have, as the earth’s shadow advanced on its
    disk, an unexpectedly misty and indistinct appearance, which lasted up to
    and including total obscuration. Golden yellow, yellow copper, dull copper,
    ruddy copper, and dull red were successively the principal colours observed
    at different times and at various portions of the moon’s surface.</p>

    <p></p>

    <div class="sidenote">As shadow
    passed off,
    indistinctness
    gave
    way to a
    sharpness of
    the moon’s
    features as
    seen through
    shadow. Two patches
    of crimson
    light described.</div>

    <p>After referring to some spectroscopic appearances, my notes then ran on
    thus:—“As the shadow began to pass off, and the bright sharp crescent of the
    illuminated portion of the moon to appear, the general aspect of the moon’s
    disk seemed to me to greatly change. The certain amount of indistinctness
    noticeable during approach and continuance of totality, gave way to a considerable
    sharpness of the moon’s features as seen through the shadow. The
    shadowed part glowed with a richer copper tint, on which were seen dark,
    almost black, spots and patches.” Then follows a description of these; and
    the notes continue:—“Two features here struck me—the one a continuation
    of the upper limb of the illuminated crescent, so that it seemed to form a
    bead of light just on the centre of the upper edge of the moon; the other
    <i>two patches of crimson light</i>, similar to those I described as having been seen
    in the last total eclipse. One of these, quite a small one, was just under the
    elongated bead before described; the other, a much larger and more diffused
    one, was seen towards the south-west limb of the moon, about midway between
    it and the centre. The spots or patches were of a decidedly crimson-red,
    in contrast to the ordinary copper-red of the disk, and were noticed by
    my friend as well as by myself.”</p>

    <div class="sidenote">Patches well
    seen in field-glass;
    lost in
    small refractor. They gradually
    deepened
    in tint.</div>

    <p>These were eye observations. The patches were quite well seen (but not
    so brightly as with the eye) with a double achromatic field-glass. With a 3¼-inch
    Cooke refractor and low power, they seemed lost in the general moon
    tint; but they were then diminishing in brightness. From a comparison of
    my two sketches, the patches seem to have gradually deepened in tint, and
    we considered them to have disappeared in a like gradual manner.</p>

    <div class="sidenote">Two
    sketches
    taken.</div>

    <p>My first sketch was taken shortly after end of total phase; the second
    about ten minutes later. I have reproduced the original sketches in preference
    to any drawing prepared from them (Plate IV. figs. 2 and 3).</p>

    <p>The patches did not last long, but were lost as the shadow swept off
    the moon. I saw nothing of the sort during the approach of or pending
    totality, nor until a small crescent of the moon began to appear behind the
    shadow.</p>

    <p>I have looked for other accounts of these patches, but cannot find any.
    Most observers have described the deeper colour of the shadowed moon by
    the word “copper.” Some extend this colour to red; but there is probably
    much in the state of the atmosphere affecting this.</p>

    <div class="sidenote">Dec. 3, 1703,
    moon’s colour
    described.</div>

    <p>At Avignon, December 3, 1703, the moon appeared, pending eclipse,
    “extraordinarily illuminated and of a very bright red,” while other and
    different features were seen at Montpelier.</p>

    <p></p>

    <p>On March 19, 1848, observers in England, Ireland, and Belgium described
    the moon’s disk as “intensely bright coppery red.” On the occasion of
    August 23-24, 1877, before mentioned, an article in one of the public papers
    described the moon’s disk, during totality, as of a “dull copper colour.”</p>

    <div class="sidenote">Mr. Keye’s
    observation.</div>

    <p>Mr. Henry Keye, in the Engadine, at a height of 4500 feet above sea-level,
    and with the purest air, saw the partially covered moon (before totality) as a
    “dull copper colour.”</p>

    <div class="sidenote">Prof. Pritchard’s. M. Faye’s. Dr. Allnatt’s
    at Frant.</div>

    <p>Prof. Pritchard, writing from the Oxford University Observatory, says that
    at 12h 10m (about the time my sketches were taken) there was a good deal of
    light on the moon’s following limb, and the colour was “more red than copper,”
    and apparently redder than it had been at a similar distance of time before
    totality. Mons. Faye reported to the French Academy of Sciences that “a
    striking phenomenon not previously noticed was that the reddish tinge, resembling
    that of a fine sunset, was deepest at the margin of the disk, a circumstance
    which he could not explain.” Dr. Allnatt, writing from Frant, says:—“At
    totality the moon’s disk presented a most extraordinary appearance: the
    western limb was comparatively transparent, but the main body appeared as
    though enveloped in a semi-opaque clot of coagulated blood, through which
    the lunar features were dimly visible.”</p>

    <div class="sidenote">Observations
    as to
    the patches.</div>

    <p>The observations of Prof. Pritchard and Mons. Faye point more immediately
    to redness; and this is the nearest approach I can find to the patches
    I noticed. These patches do not seem to me easy of explanation. They could
    not well be colours or details due to the actual surface of the moon itself. The
    moon, we are aware, has only a certain portion of the visible disk slightly tinted.
    The Mare Serenitatis is certainly of a slight green tinge; and to the Palus
    Somni and certain other districts is attributed a pale red or pink; but these
    tints could hardly have sufficed to produce the effect seen, as the patches were
    conspicuous for a bright and decided colour. The positions, moreover, did
    not correspond; while the ease with which other details of the surface were
    seen at the time would, if the tints had arisen from the surface itself, probably
    have enabled the circumstance to be detected.</p>

    <div class="sidenote">Refraction
    of sun’s rays
    not a satisfactory
    explanation.</div>

    <p>The refraction of the sun’s rays by passage through the earth’s atmosphere
    is, too, not a satisfactory explanation. This, as judged by the appearance of
    the covered moon immediately before and at totality, gives a disk of shadow
    deeper in tone in the centre and lightening towards the edges, but in other
    respects fairly uniform, so that the whole disk seems to partake of the same
    tint and its graduations; and this is what might have been expected under
    the circumstances. The patches, on the other hand, were quite local.</p>

    <p></p>

    <div class="sidenote">Question of
    lunar atmosphere.</div>

    <p>The theory of the moon’s possessing no atmosphere whatever is now
    very generally, but perhaps too readily, received (mainly upon the evidence
    of the spectroscopic observations of occulted stars&nbsp;<span class="footnote">The proof from occulted stars merely goes to the fact that the moon possesses no atmosphere
  <i>appreciable in that way</i>. It may still be a question whether there does not exist something
  of the kind, lying low and close to the surface, and possibly of a rarefied character, which would
  scarcely make itself visible by its effects in occultations. Cloud-vapour might form in an
  atmosphere of inconsiderable density.</span>), as there still seems a
    reasonable doubt whether our satellite may not possess an atmosphere, possibly
    rarefied, but yet sufficiently dense to permit of the formation of cloud or
    vapour.</p>

    <div class="sidenote">Instance of
    patch of
    vapour or
    cloud on
    moon’s surface.</div>

    <p>A curious case, in which a patch of vapour or cloud was supposed to be
    detected on the moon’s surface, is reported by the Rev. J. B. Emmett in a
    communication to the ‘Annals of Philosophy’ (New Series, vol. xii. p. 81).
    It is dated “Great Ouseburn, near Boroughbridge, July 5, 1826,” the observation
    being made with “the greatest care with a very fine telescope.”</p>

    <p>On the 12th April 8h, while observing the part of the moon called Palus
    Mœotis by Nevelius, with an excellent Newtonian reflector of 6 inches
    aperture, at a particular part of the Palus, which he minutely describes, he
    saw, with powers 70 and 130, “a very conspicuous spot wholly enveloped in
    black nebulous matter, which, as if carried forward by a current of air,
    extended itself in an easterly direction, inclining a little towards the south,
    rather beyond the margin of Mœotis.” April 13th 8h to 9h, the cloudy appearance
    was reduced both in extent and intensity, and the spot from which
    it seemed to issue had become more distinctly visible. On April 17th
    scarcely a trace of the nebulous matter remained; but so long after as
    June 10th 8h “a little blackness” remained about the spot. Mr. Emmett
    suggested “smoke of a volcano or cloudy matter.” A copy of the drawing
    annexed to the paper is given on Plate X. fig. 10 (black patch on moon).
    If this observation was (as it certainly appears to be) critical and exact, there
    must have been a disturbance of the moon’s surface, indicating some sort of
    cloud- or vapour-supporting atmosphere; and probably, for the purposes of
    Auroræ, an atmosphere of a very rarefied condition would suffice&nbsp;<span class="footnote">This observation is not without a certain amount of confirmation by more recent ones, in
  which certain lunar objects and regions have been suspected of mist or vapour. Mr. Birt
  (‘English Mechanic,’ vol. xxviii. no. 725) mentions two—the cloud-like appearance of the white
  patch west of Picard, and the interior of Tycho, which at one time always misty and ill-defined,
  is now become perfectly distinct and sharply defined.<br/>December 4, 1878, 4h 45m. I observed Klein’s crater as a dull dark spot, larger than the true
  object; and while definition was good and other objects were well defined, “the floor of Klein’s
  object, the oval spot near, and also Agrippa (especially), all had <i>an odd misty look as if vapour
  were in or about them</i>” (‘English Mechanic,’ vol. xxviii. no. 727). The mystery of different
  observers seeing and not seeing Klein’s object on the same night is hardly to be accounted for
  by the angle of illumination.</span>.</p>

    <p></p>

    <div class="sidenote">Prof. Alexander’s
    evidence
    in
    favour of a
    lunar atmosphere.</div>

    <p>According to the ‘New York Tribune,’ at a recent semi-annual meeting of
    the American Academy of Sciences, Professor Alexander “brought forward
    a variety of evidence tending to indicate some envelope like an atmosphere
    for the moon. The evidence was principally drawn from observations during
    eclipses. The explanations usually offered for the bright band seen around
    the moon at such times was fully considered, and shown to be inadequate,
    though good as far as they would apply. The ruddy band of light is much
    too broad to be the sun’s chromosphere. It was most apparent in those
    instances where the moon was nearest the earth. It would best be accounted
    for by supposing an atmosphere to the moon, a thin remnant of ancient
    nebulosity, comparable to that which accompanies the earth and gives rise to
    the appearance of the Aurora Borealis.” Is it not, however, possible that
    the appearance might have arisen from Auroræ in action within the region
    of the earth’s own atmosphere during the passage of the sun’s rays through
    it at the time of the eclipse? The whole subject is difficult of explanation,
    and should be one of the points for attention on the occasion of the next
    total Lunar eclipse. It seemed to me appropriate for introduction into the
    present history of the Aurora, whatever its solution may ultimately be.</p>

    <div class="sidenote">Mars and
    Jupiter.</div>

    <p>In the case of Mars and Jupiter, whose atmospheres are sufficiently recognized,
    red- and scarlet-tinted patches are frequently noticed. In Mars this is
    generally attributed to the geological character of the surface of the planet
    itself; but I have observed on Mars’s surface during the recent opposition a
    local rosy tint of a more diffused and indefinite character; and in the case of
    Jupiter the appearances seem almost always connected with the clouds’ belts,
    as distinguished from the regions lying nearer to the planet’s surface.</p>

    <div class="sidenote">Prof.
    Dorna’s
    “Lunar
    Aurora.”</div>

    <p>Professor Dorna, of Turin, ascribed a flickering light seen on the reddened
    disk of the moon during the Lunar eclipse of February 1877 to the action of
    a <i>Lunar Aurora</i>, holding that the refraction of the sun’s rays within the
    cone of the earth’s shadow was not an adequate explanation (‘L’Opinione
    Nazionale,’ March 3, 1877).</p>

    <div class="sidenote">Spectroscopic
    observations
    bearing on
    the subject. Mr. Christie’s
    observations
    at
    Greenwich.</div>

    <p>The spectroscope might have afforded some information on the question;
    but my own telescopes (8¼ and 3¼ in.) were not of sufficient aperture to
    give a sensible spectrum of a portion of the moon’s eclipsed surface, and my
    observations were chiefly made on the entire disk with hand-spectroscopes
    without a slit. Mr. Christie, at the Royal Observatory, Greenwich, made a
    set of observations during totality, and also during subsequent partial phase,
    with a single-prism spectroscope. During totality a strong absorption band
    was seen in the yellow, and the red and blue ends of the spectrum were
    completely cut off, while the orange was greatly reduced in intensity. The
    yellow and green were comparatively bright, and seemed to constitute the
    whole visible spectrum. The absorption band became narrowed as the end
    of the total phase approached, and during partial phase was reduced to a
    mere line. The red end of the spectrum was cut off by a dark band commencing
    about halfway from D to C, in which a black line was suspected.
    The bands observed were characteristic of the spectrum of light which has
    passed through a thick stratum of air. In the description of the spectrum
    of the Aurora in Part II., it will be seen that the conspicuous red and green
    lines of the Aurora are either coincident with, or very close to, some of these
    atmospheric lines. It does not appear that Mr. Christie examined the
    crimson patches specifically, nor that he saw bright lines on any part of the
    moon’s eclipsed disk.</p>

    <div class="sidenote">Mr. Pratt’s
    notes of
    Lunar
    Eclipse,
    August 23,
    1877.</div>

    <p>Mr. Henry Pratt has also kindly handed me for use his notes of the Lunar
    eclipse of August 23, 1877, as seen at Brighton on a splendid night. They
    were made as the phenomenon progressed, are 58 in number, and in many
    instances only a few minutes, or even seconds, apart. A selection of them is
    here given:—9h 13m 50s, first contact of shadow. 9h 30m, shadow very dark;
    no details of disk easily seen. 9h 40m, first appearance of red. 9h 50m, <i>red</i>
    all over disk, except margin bluish and S. part green tint. 10h 2m, <i>a sudden
    brightening of whole disk</i>, in strong contrast to two minutes previously.
    10h 15m, <i>E. limb much darker</i>. 10h 35m, <i>south pole decidedly brightest</i>. 10h 44m,
    <i>S.E. limb much brighter</i>. 10h 48m, <i>whole disk much darker</i>. 10h 51m, <i>S.E.
    limb brightening again</i>. 11h 1m, <i>N.E. limb brightening</i>. 11h 3m, <i>N.E. limb
    has darkened and brightened three times during last two minutes</i>. 11h 20m,
    N. pole has <i>darkened</i>. 11h 21m, N. pole has <i>brightened</i>. 11h 24m 30s, N. pole
    darker <i>red</i>. 11h 35m, N. pole <i>bright</i>. 11h 35m 30s, same <i>dark</i> and <i>red</i>.
    11h 42m, N.E. limb especially bright for a few seconds, and then <i>reddened</i>
    and shaded again. 11h 49m, <i>S. pole reddened</i>. 12h 1m, <i>S.W. limb reddest
    part; S. pole red; N. pole paler red</i>. 12h 3m 50s, first appearance of E. limb
    (my first sketch was made shortly after this, and my second about ten minutes
    later). 12h 21m, a bright patch on N.N.W. separated from N. pole. 12h 24m,
    <i>S.W. region is reddest part of eclipse</i>. 12h 40m, <i>redness</i> of shadow fading out.</p>

    <p>With a small Browning star-spectroscope Mr. Pratt saw the red and blue
    ends of the spectrum cut off, but nothing else. Mr. Pratt adds that the
    <i>red</i> colour was not an effect of contrast or an optical delusion in any way, as
    was proved by using at times a limited field containing only the red portion
    under examination. In reference to the curious brightening and darkening
    of the disk, and the change from time to time of local colour, he says that
    with much experience he has seen nothing of the same marked character on
    other occasions, and that “the whole matter was at the time astonishing to
    me, but none the less real.” The local red patches seen by me seem also to
    have been observed by Mr. Pratt.</p>

    <div class="sidenote">Mr. Pratt’s
    observation
    on the floor
    of Plato.</div>

    <p>As an addition to the instances of Tycho, Picard, &amp;c., mentioned in the
    note on p. 73, Mr. Pratt has also sent me his notes of some observations by
    him, of “local obscuration of the floor of Plato.” As somewhat condensed,
    they are as follows:—1872, July 16. While in other parts of the floor spots
    and streaks were well visible, “the N.W. portion was in such a hazy condition
    that nothing could be defined upon it.” 1873, Nov. 1. 27 light
    streaks seen (7 new): the brightness of the streaks was in excess of their
    usual character, as compared with the craterlets; “an <i>obliteration</i> or <i>invisibility</i>
    of <i>all</i> the light streaks in the neighbourhood of craterlet no. 1 was very
    noticeable;” and also “a similar obliteration of the N. end of the streak
    called the Sector, near craterlet 3.” 1874, January 1. 18 light streaks seen,
    including 3 new, “some of which outshone other longer known ones. This
    was curious; for had they been as bright within the last two years as on this
    occasion I must have noticed them.” Mr. Pratt points out, as worthy of
    remark, that some months previous to November 1st, 1873, neither craterlets
    nor streaks on the floor of Plato “had maintained their previous characteristic
    brightness,”—a fact which he thinks ought to be considered together
    with the outbreak of brilliancy of both orders on that day, as well as
    the apparently sudden existence of new ones.</p>

    <div class="sidenote">Observation
    by Mr. Hirst
    of a dark
    shade on the
    moon.</div>

    <p>The ‘Observatory,’ March 1, 1879, p. 375, contains an account, by Mr.
    H. C. Russell, of some Astronomical Experiments made on the Blue Mountains,
    near Sydney, N. S. W. Among these it is noticed that on 21st October, 1878, at
    9 <span class="smcapuc">A.M.</span>, when looking at the moon, Mr. Hirst found that a large part of it was
    covered with a dark shade, quite as dark as the shadow of the earth during
    an eclipse of the moon. Its outline was generally circular, and fainter near
    the edges. Conspicuous bright lunar objects could be seen through it; but it
    quite obliterated the view of about half the moon’s terminator, while those
    parts of the terminator not in the shadow were distinctly seen.</p>

    <p>No change in the position of the shade could be detected after three hours’
    watching. The observation is made, “One could hardly resist the conviction
    that it was a shadow; yet it could not be the shadow of any known
    body. If produced by a comet, it must be one of more than ordinary
    density, although dark bodies have been seen crossing the sun which were
    doubtless comets.” The diameter of the shadow from the part of it seen on
    the moon was estimated at about three quarters that of the moon&nbsp;<span class="footnote">Some doubt has been cast on this observation, on the ground that nothing unusual was
  seen, and that the appearances were only those ordinarily presented by the moon at its then
  phase. I simply give the account as it appears in the scientific journal in which it was
  published.</span>.</p>

  </section>

  <!-- Chapitre 8_________________________________________________________-->
  <section class="chapter" id="chapter-8">
    <h3 class="titlechapter" id="chap-8">Aurora and Solar Corona</h3>
    <p class="shorter">Aurora and Solar Corona</p>

    <div class="sidenote">Aurora and
    the solar
    corona. Mr. Norman
    Lockyer’s
    ‘Solar
    Physics.’</div>

    <p>Mr. Norman Lockyer, in his ‘Solar Physics,’ a work of 666 pages, gives but
    little space to the Aurora. The index comprises:—“Aurora Borealis, connexion
    with sun-spots, pp. 82-102.” “Affirmed coincidence of spectrum
    with that of the corona, pp. 244, 256.”</p>

    <div class="sidenote">Extracts
    from as to
    Aurora’s
    connexion
    with sun-spots
    and
    with solar
    corona.</div>

    <p>Page 82. After referring to Gen. Sabine as having shown that there are
    occasional disturbances in the magnetic state of the earth, and that these
    disturbances have a periodical variation, coinciding in period and epoch with
    the variation in frequency and magnitude of the solar spots as observed by
    Schwabe, the author proceeds to state, “and the same philosopher has given
    us reason to conclude that there is a similar coincidence between the outburst
    of solar spots and of the Aurora Borealis.”</p>

    <p>Page 102. “We have also shown that sun-spots or solar disturbances
    appear to be accompanied by disturbances of the earth’s magnetism, and these
    again by auroral displays.”</p>

    <div class="sidenote">Evidence of
    American
    observers on
    nature of the
    corona considered.</div>

    <p>Page 243. “What, then, is the evidence furnished by the American
    observers on the nature of the corona (solar)? It is bizarre and puzzling to
    the last degree. The most definite statement on the subject is that it is
    nothing more nor less than a <i>permanent Solar Aurora</i>! the announcement
    being founded on the fact that three bright lines remained visible after the
    image of a prominence had been moved away from the slit, and that one
    (if not all) of these lines is coincident with a line (or lines) noticed in the
    spectrum of the Aurora Borealis by Professor Winlock.” Mr. Lockyer then
    adds, that amongst the lines he had observed up to that time, some forty in
    number, this line was among those which he had most frequently recorded,
    and was, in fact, the first iron line which made its appearance in the part of
    the spectrum he generally studied, when the iron vapour is thrown into the
    chromosphere.</p>

    <div class="sidenote">Mr. Lockyer’s
    conclusion
    adverse
    to the question
    being
    settled.</div>

    <p>Hence he thought he should always see it if the Aurora were a permanent
    solar corona, and gave out this as its brightest line, and on this ground alone
    should hesitate to regard the question as settled.</p>

    <div class="sidenote">Prof.
    Young’s
    communication
    to
    ‘Nature.’</div>

    <p>Page 256 is an extract from a communication by Prof. Young to ‘Nature,’
    March 24, 1870, in which the Professor refers to the bright line 1474 as
    being always visible with proper management. He also thinks it probable
    that this line coincides with the Aurora line reported by Prof. Winlock
    at 1550 of Dr. Huggins’s scale, though he is by no means sure of it. He had
    only himself seen it thrice, and then not long enough to complete a measurement.
    He was only sure that its position lay between 1460 and 1490 of
    Kirchhoff.</p>

    <div class="sidenote">He does not
    abandon his
    hypothesis,
    it having
    other elements
    of
    probability.</div>

    <p>For this reason he did not abandon the hypothesis, which appeared to have
    other elements of probability, in the general appearance of the corona, the
    necessity of immense electrical disturbances in the solar atmosphere as the
    result of the powerful vertical currents known to exist there, as well as
    the curious responsiveness of our terrestrial magnets to solar storms; yet he
    did not feel in a position to urge it strongly, but rather awaited developments.
    Father Secchi was disposed to think the line hydrogen, while
    Mr. Lockyer still believed it to be iron.</p>

    <div class="sidenote">Dr. Schellen
    reviews the
    subject
    in eclipse
    of 1869.</div>

    <p>Dr. Schellen, in his ‘Spectrum Analysis,’ treats the matter more in detail.
    Referring to the eclipse of 1869 as confirming the previous observations
    that the coronal spectrum was free from dark lines, he points out that
    Pickering, Harkness, Young, and others were agreed that with the extinction
    of the last rays of the sun all the Fraunhofer lines disappeared at once
    from the spectrum. He further says:—</p>

    <div class="sidenote">Young observed
    three
    bright lines
    in the spectrum
    of the
    corona. Coincidence
    of these lines
    with three
    bright lines
    observed by
    Winlock in
    the Aurora. Corona self-luminous,
    and probably
    of a gaseous
    nature. Corona supposed
    to be
    a permanent
    polar light
    existing in
    the sun. Polar light
    in the sun
    attributed to
    electricity. Dr. Schellen
    thinks
    nature of the
    corona still
    a problem.</div>

    <p>“The small instruments employed by Pickering and Harkness, with a large
    field of view, exhibited a spectrum obtained at once from the corona, the prominences,
    and the sky in the neighbourhood of the sun. These instruments
    showed during totality a faint continuous spectrum free from dark lines, but
    crossed by two or three bright lines. Young, with a spectroscope of five
    prisms, observed the three bright lines in the spectrum of the corona, and
    deduced the following positions according to Kirchhoff’s scale:—1250 ± 20,
    1350 ± 20, and 1474. It had been already explained why the last and
    brightest of these lines was thought to belong to the corona and not to that
    of the prominences, and it seemed probable that the other two lines belonged
    also to the light of the corona, from the fact that they were both wanting in
    the spectrum of the prominences when observed without an eclipse. But
    what invested these three lines with a peculiar interest was the circumstance
    that they appeared to coincide exactly with the first three of the five bright
    lines observed by Professor Winlock in the spectrum of the Aurora Borealis.
    These lines of the Aurora were determined by Winlock according to Huggins’s
    scale; and if these be reduced to Kirchhoff’s scale, the positions of the lines
    would be 1247, 1351, and 1473, while the lines observed by Young were 1250,
    1350, and 1474.” Dr. Schellen then points out that if it be borne in mind
    that Young found the positions of the two fainter lines more by estimation
    than by measurement, the coincidence between the bright lines of the corona
    and those of the Aurora would be found very remarkable. The brightest of
    the lines, 1474, was the reversal of a strongly marked Fraunhofer line,
    ascribed by Kirchhoff and Ångström to the vapour of iron. Dr. Schellen
    then details Professor Pickering’s observations with the polariscope, showing
    that the corona must be self-luminous, and that from the bright lines seen in
    its spectrum it is probably of a gaseous nature, and forms a widely diffused
    atmosphere round the sun; and then adds, “It has been supposed, from the
    coincidence of the three bright lines of the corona with those of the Aurora
    Borealis, that the corona is a permanent polar light existing in the sun
    analogous to that of our earth.” Dr. Schellen here adds:—“Lockyer, however,
    justly urges against this theory the fact that although the brightest of
    these three lines, which is due to the vapour of iron, is very often present
    among the great number of bright lines occasionally seen in the spectrum of
    the prominences, it is by no means constantly visible, which ought to be the
    case if the corona were a permanent polar light in the sun.” (Professor
    Young’s answer to this, on the ground of line 1474 being always visible,
    has been already given.) “A yet bolder theory is the ascription of such a
    polar light in the sun to the influence of electricity, which has been proved,
    it is well known, by the relation of the magnetic needle, and the disturbance
    of the electric current in the telegraph wires, to play an important part in
    the phenomenon of the Aurora Borealis;” and Dr. Schellen then concludes with
    an opinion that the nature of the corona was still a problem&nbsp;<span class="footnote">The question of a connexion between the waxing and waning of the solar corona and the
  prevalence of sun-spots is now being mooted, and may have an important bearing on the subject
  of the constitution of the corona. It would seem that when the corona has been examined about
  the time of minimum of sun-spots, it has proved fainter though more extended, while the bright
  lines of the spectrum have been absent, indicating a change or variance in the gaseous part of it
  at those periods.</span>.</p>

    <div class="sidenote">Various
    places of
    wave-length
    assigned to
    these lines.</div>

    <p>On reference to the ‘American Journal of Science,’ vol. xlviii. pp. 123 and
    404, it seems that the auroral observations before referred to were made on
    15th April, 1869, by C. S. Pierce, with “an ordinary chemical spectroscope,
    with the collimator pointed directly to the heavens,” and were reported by
    Winlock. The lines were 1280, 1400, and 1550 of Huggins’s scale, and were
    reduced to Kirchhoff’s scale by Young. These lines have had all sorts of
    places of wave-length assigned to them by different writers. Proctor gives
    5570, 5400, 5200; Pickering and Alvan Clarke, 5320 (assumed to be 5316,
    coronal line); Barker, 5170, 5200, 5020; Backhouse, 5320, 4640, and 4310.
    In my ‘Aurora Spectrum,’ Plate XII., I have assigned two, with a?, to 5320
    (Alvan Clarke) and 5020 (Barker). The third might perhaps be placed at
    4640 (Backhouse and Winlock).</p>

    <div class="sidenote">Doubts
    raised as to
    closeness
    of the observations
    for the purpose
    of
    comparison.</div>

    <p>The coincidences relied on in the foregoing observations depend, of course,
    upon (1) the accuracy of the observations themselves, and (2) the subsequent
    reduction of the lines for comparison. Assuming the correctness of the latter,
    what have we as to the former? Two of Professor Young’s positions of
    coronal lines, as stated, seem to have far too much of the ± element to make
    them sufficiently accurate. Pierce’s auroral observation does not state how
    the lines were positioned. As they <i>all</i> end with a cypher, the suspicion
    naturally arises that the measurements did not extend beyond the first three
    places of the figures, and, if so, could not be used for accurate comparison.
    The auroral lines, too, are generally rather wide and nebulous, and not easy
    of comparison with sharper ones.</p>

  </section>

  <!-- Chapitre 9_________________________________________________________-->
  <section class="chapter" id="chapter-9">
    <h3 class="titlechapter" id="chap-9">Supposed Causes of the Aurora</h3>
    <p class="shorter">Supposed Causes of the Aurora</p>

    <div class="sidenote">Supposed
    causes of the
    Aurora. Sulphurous
    vapours. Magnetic
    effluvia.</div>

    <p>At first the Aurora was described to be sulphurous vapours issuing from the
    earth; and Musschenbroek pointed out that certain chemical mixtures sent
    forth a phosphorescent vapour, in some respects resembling the Aurora. Dr.
    Halley originally proposed a similar theory, but ultimately concluded that
    the Aurora might be occasioned by the circulation of the magnetic effluvia of
    the earth from one pole to another.</p>

    <div class="sidenote">Zodiacal
    light.</div>

    <p>M. de Mairan, in 1721, in a treatise, ascribed the Aurora to the impulse
    of the zodiacal light upon the atmosphere of the earth.</p>

    <div class="sidenote">Luminous
    particles of
    our atmosphere.</div>

    <p>Euler combated this theory, and ascribed the Aurora to the luminous
    particles of our atmosphere driven beyond its limits by the light of the sun,
    and sometimes ascending to the height of several thousand miles.</p>

    <div class="sidenote">Electric fluid
    <i>in vacuo</i>
    resembles
    Aurora.</div>

    <p>Mr. Hawksbee very early showed that the electric fluid assumes, <i>in vacuo</i>
    or in highly rarefied atmosphere, an appearance resembling the Aurora. Mr.
    Canton contrived an imitation of the Aurora by means of electricity transmitted
    through the Torricellian vacuum in a long glass tube, and showed
    that such a tube would continue to display strong flashes of light for 24
    hours and longer without fresh excitation.</p>

    <div class="sidenote">Experiment
    with electrical
    machine
    and exhausted
    receiver.</div>

    <p>In the ‘Edinburgh Encyclopædia,’ date 1830, is mentioned an experiment
    in which an electrical machine and air-pump are so disposed that strong sparks
    pass from the machine to the receiver of the air-pump.</p>

    <div class="sidenote">Dr. Franklin’s
    theory.</div>

    <p>As the exhaustion proceeds the electricity forces itself through the receiver
    in a visible stream, at first of a deep purple colour; “but as the exhaustion
    advances it changes to blue, and at length to an intense white, <i>with which
    the whole receiver becomes completely filled</i>.” [It will be noticed that this
    experiment bears a close resemblance to Prof. Ångström’s exhausted flask
    referred to later in treating of the spectrum of the Aurora.]</p>

    <p>Dr. Franklin gave a different form to the electric theory of the Aurora,
    supposing that the electricity which is concerned in the phenomenon passes
    into the Polar regions from the immense quantities of vapour raised into the
    atmosphere between the tropics (Exper. and Observ. 1769, p. 43).</p>

    <div class="sidenote">Mr. Kirwan’s
    theory.</div>

    <p>Mr. Kirwan (Irish Trans. 1788) supposed that the light of the Aurora
    Borealis and Australis was occasioned by the combustion of inflammable air
    kindled by electricity.</p>

    <div class="sidenote">Mons.
    Monge’s.</div>

    <p>Mons. Monge proposed the theory that the Auroræ were merely clouds
    illuminated by the sun’s light falling upon them after numerous reflections
    from other clouds placed at different distances in the heavens (Leçons de
    Physique par Prejoulz, 1805, p. 237).</p>

    <span class="sidenote">Mons. Libes’.</span>

    <p>Mons. Libes propounded a theory that the electric fluid, passing through a
    mixture of azote and oxygen, produced nitric acid, nitrous acid or nitrous
    gas, and that these substances, acted upon by the solar rays, would exhibit
    those red and volatile vapours which form the Aurora Borealis (Traité de
    Physique, ou Dictionnaire de Physique, par Libes; Rozier’s Journal, June
    1790, February 1791, and vol. xxxviii. p. 191).</p>

    <span class="sidenote">Mr. Dalton’s.</span>

    <p>Mr. Dalton considered the Aurora a magnetic phenomenon whose beams
    were governed by the magnetism of the earth. He observed that the luminous
    arches were always perpendicular to the magnetic meridian (Dalton’s
    Meteorological Observations and Essays, 1793, pp. 54, 153).</p>

    <span class="sidenote">Abbé Bertholon’s.</span>

    <p>The Abbé Bertholon ascribed the Aurora Borealis to a phosphorico-electric
    light (Encyc. Méthod. art. Auroræ).</p>

    <div class="sidenote">Dr. Thompson
    concluded
    the
    arches to be
    an optical
    deception.</div>

    <p>Dr. Thompson (Annals of Philosophy, vol. iv. p. 429), from the observations
    of Mr. Cavendish and Mr. Dalton, concluded there was no doubt that
    the arched appearance of the Aurora was merely an optical deception, and
    that in reality it consisted of a great number of straight cylinders parallel
    to each other and to the dipping-needle at the place where they were seen.</p>

    <div class="sidenote">Artificial
    Auroræ produced
    in exhausted
    tubes.</div>

    <p>With many of us (at least it was so in my own case) our first viewed Auroræ
    have been artificial ones, devised by electricians and having their locus at the
    Royal Polytechnic in Regent Street or in some scientific lecture-room. The
    effects in these cases are produced in tubes nearly exhausted by means of an
    air-pump, and then illuminated by some form of electric or galvanic current.</p>

    <span class="sidenote">Tubes described.</span>

    <p>In one instance the tube is usually of the form shown on Plate X. fig. 9,
    supported on a base with a brass ball electrode at the lower end, and a pointed
    wire at the upper. In another case the tube is of the form shown on same
    Plate, fig. 8. After exhaustion it is permanently closed, the current passing
    through it by means of the platinum-wire electrodes introduced into each
    end of the tube. The first form of tube is usually excited by a frictional
    plate machine; the second by a galvanic current from a Grove or bichromate
    battery, which, by the aid of a Ruhmkorff coil, has had its character changed
    from quantity to intensity. In each instance, upon connexion with the source
    supply of the electric current, a very similar effect is produced.</p>

    <p></p>

    <span class="sidenote">Effects described.</span>

    <p>Brilliant streams of rose-coloured light pass between the electrodes, sometimes
    as a single luminous misty band, sometimes in divided vibrating sprays
    or streams, and sometimes in a flaky column of striæ.</p>

    <p>All this, before the spectroscope took its part in the investigation, we were
    content to accept as a very fair and probable explanation of the Aurora accompanied
    by a mimic representation of the phenomenon.</p>

    <p>These appearances may, of course, be produced at will in tubes having
    electrodes; but it is, moreover, possible to produce them, though with less
    effect, in certain other forms of tube having no such direct communication
    with the external electric machine.</p>

    <p>One electrode only may be connected with the coil or electrical machine.
    The appearance is then a faint representation of what happens when the
    current entirely passes (but see experiments with a single wire detailed in
    Part III.).</p>

    <div class="sidenote">Tube without
    electrodes.</div>

    <p>In the case of an exhausted tube having no electrodes, the wires from the
    coil may be made into a little helix and placed at each end of the tube, and
    the induced currents within will show themselves in flashes and streams of
    light, varying in colour and tint according to the gaseous or other contents of
    the tube.</p>

    <div class="sidenote">Tube excited
    by friction.</div>

    <p>In some cases the ordinary forms of galvanic or electrical machine for supplying
    the current of electricity may be dispensed with. A long straight tube
    exhausted and closed at each end, and without electrodes, Plate X. fig. 6, being
    slightly warmed and then excited by friction with the dry hand or a piece of
    flannel, silk handkerchief, or the like, is soon filled with the most brilliant
    flashes of light playing in the interior, and when once thoroughly charged
    needs but little further excitation to keep up the effect.</p>

    <div class="sidenote">Geissler’s
    mercury
    tube.</div>

    <p>Geissler has introduced a form of tube in which electricity in its form of
    flashes and glow of light is produced by the friction of mercury. The outer
    tube is strong, and contains within it a smaller tube of uranium glass with
    balls blown upon it (Plate X. fig. 7). The tubes are exhausted and a small
    quantity of mercury is introduced which has access to both surfaces of the
    inner tube, as well as to the inner surface of the outer tube. Upon the tube
    being reversed end for end or shaken, the mercury runs up or down the tube
    and causes a very considerable display of whitish light.</p>

    <p>The before-described tubes are also referred to, and their spectra described,
    in the section “On the comparison of some tube and other Spectra with the
    Aurora” (Part II.).</p>

    <p>The aura or brush from the electrical machine has been considered as
    resembling the Aurora, while the hissing and crackling accompanying it has
    been supposed to corroborate the reports of similar noises having been heard
    during an auroral display.</p>

    <div class="sidenote">Prof. Lemström’s
    instrument
    to
    demonstrate
    the nature
    of Auroræ.</div>

    <p>Prof. Lemström, of the University of Helsingfors, has devised an instrument
    for the purpose of demonstrating that Auroræ are produced by electrical
    currents passing through the atmosphere. An illustration of this instrument
    (for which I am indebted to the Editor of ‘Nature’) is introduced (fig. 1).</p>

    <p>The instrument was exhibited at the recent Scientific Loan Collection at
    South Kensington, and a full description of it, together with an essay by
    Prof. Lemström, “On the Theory of the Polar Light,” will be found in the
    third edition of the Official Catalogue, p. 386. no. 1751. The apparatus is
    intended to show that an electric current passing from an insulated body does
    not produce light in air of normal pressure; but as it rises to the rarefied air
    in the Geissler tubes a phenomenon very like the real Polar Light is produced.</p>

    <div class="figcenter" style="width: 400px;">
    <!-- <img src="assets/aurorae/images/fig1.jpg" width="400" height="500" alt="" /> -->
    <p class="caption">Fig. 1.</p>
    </div>

    <p>A is an electrical machine, the negative pole being connected with a copper
    sphere and the positive with the earth.</p>

    <p><i>s s´</i> is of ebonite as well as R R <i>d</i>, so that B is quite insulated as the earth
    is in space. B is surrounded by the atmosphere. <i>a´ a´ a´ a´ a´ a´</i> are a series of
    Geissler tubes with copper ends above and below. All the upper ends are
    connected with a wire which goes to the earth; consequently a current runs
    in the direction of the arrows through the air, and the Geissler tubes become
    luminous when the electrical machine is set into operation.</p>

    <p>The Geissler tubes represent the upper part of the atmosphere which
    becomes luminous when the Aurora Borealis is observed in the northern
    hemisphere. The phenomena produced by the Lemström apparatus are
    considered consistent with the theory advocated by Swedish observers that
    electrical currents emanating from the earth and penetrating into the upper
    regions produce Auroræ in both hemispheres. The experiment differs from
    the apparatus of M. de la Rive, who placed his current <i>in vacuo</i>, and did not
    show the property of ordinary atmospheric air, in allowing to pass unobserved,
    at the pressure of 760 millims., a stream of electricity which illuminates a
    rarefied atmosphere.</p>

    <div class="sidenote">M. de la
    Rive’s apparatus
    described.</div>

    <p>De la Rive’s apparatus was also exhibited at the same time, and will be
    found described at p. 385 of the Catalogue, No. 1749. A large sphere of
    wood represented the earth, and iron cylinders the two extremities of the
    terrestrial magnetic axis. These penetrated into two globes filled with
    rarefied air, simulating the higher regions of the Polar atmosphere. The
    electric discharge turned around a point situate in the prolongation of the
    axis, in a different direction at either pole, when the two cylinders were
    charged by means of a horseshoe electro-magnet, in accordance with observations
    on the rotation of the rays of the Aurora.</p>

    <div class="sidenote">De la Rive’s
    magnet in
    an electric
    egg.</div>

    <p>De la Rive placed an electro-magnet in an electric egg. As soon as the
    magnet was set in action the discharge which had before filled the egg was
    concentrated into a defined band of light, which rotated steadily round the
    magnet.</p>

    <div class="sidenote">Gassiot’s
    experiment
    with 400
    Grove cells
    and exhausted
    receiver
    between
    poles of
    magnet.</div>

    <p>Gassiot describes an experiment with his great Grove’s battery of 400 cells,
    in which an exhausted receiver was placed between the poles of the large
    electro-magnet of the Royal Institution.</p>

    <p>“On now exciting the magnet with a battery of 10 cells, effulgent strata
    were drawn out from the positive pole, and passed along the under or upper
    surface of the receiver according to the direction of the current.</p>

    <p>“On making the circuit of the magnet and breaking it immediately, the
    luminous strata rushed from the positive pole and then retreated, cloud following
    cloud with a deliberate motion, and appearing as if swallowed up by
    the positive electrode.” Mr. Marsh considered this bore a very considerable
    resemblance to the conduct of the auroral arches, which almost invariably
    drift slowly southward.</p>

    <p></p>

    <div class="sidenote">Mr. Marsh
    considered
    the Aurora
    an electric
    discharge
    between the
    magnetic
    poles of the
    earth.</div>

    <p>He considered it probable that the Aurora was essentially an electric
    discharge between the magnetic poles of the earth, leaving the immediate
    vicinity of the north magnetic pole in the form of clouds of electrified matter,
    which floated southward, bright streams of electricity suddenly shooting forth
    in magnetic curves corresponding to the points from which they originated,
    and then bending southward and downward until they reached corresponding
    points in the southern magnetic hemisphere, and forming pathways by which
    the electric currents passed to their destination; and, further, that the magnetism
    of the earth caused these currents and electrified matter composing
    the arch to revolve round the magnetic pole of the earth, giving them their
    observed motion from east to west or from west to east.</p>

    <div class="sidenote">Varley’s
    observation
    on a glow-discharge
    <i>in vacuo</i>. Spark surrounded
    by
    an aura
    which could
    be separated.</div>

    <p>Varley showed that when a glow-discharge in a vacuum tube is brought
    within the field of a powerful magnet, the magnetic curves are illuminated
    beyond the electrodes between which the discharge is taking place, as well
    as in the path of the current, and also thought that this illumination was
    caused by moving particles of matter, as it deflected a balanced plate of talc
    on which it was caused to infringe. It has also been shown that in electrical
    discharges in air at ordinary pressure, while the spark itself was unaffected by
    the magnet, it was surrounded by a luminous cloud or aura which was driven
    into the magnetic curve, and which might also be separated from the spark
    by blowing upon it.</p>

    <p>Most of the foregoing interesting results and experiments will be found
    repeated and verified in Part III.</p>

    <h4 id="chap-9-1"><i>Prof. Lemström’s Theory.</i></h4>

    <div class="sidenote">Prof. Lemström’s
    theory.</div>

    <p>Prof. Lemström thinks that terrestrial magnetism plays only a comparatively
    secondary part in the phenomena of the Polar Light, this part consisting
    essentially in a direct action upon the rays.</p>

    <p>That the experiments of M. de la Rive do not all furnish the proof that the
    rays of the light are really united under this influence.</p>

    <div class="sidenote">Character
    of the Polar
    Light.</div>

    <p>That the Polar Light considered as an electrical discharge gives the following
    results:—</p>

    <p>(1) An electric current arising from the discharge itself, which takes place
    slowly.</p>

    <p>(2) Rays of light consisting of an infinite number of sparks, each spark
    giving rise to two induction currents going in opposite directions.</p>

    <p>(3) A galvanic current going in an opposite direction to that of the discharge,
    and having its origin in the electromotive force discovered by M. Edlund in
    the electric spark. That these currents require a closed circuit; but this is
    not necessary in the case of the Aurora, as the earth and rarefied air of the
    upper regions are immense reservoirs of electricity producing the same effect
    as if the circuit were closed. That permanent moisture in the air, a good
    conductor of electricity, is the cause of a slow and continuous discharge
    assuming the form of an Aurora, instead of suddenly producing lightning as
    in equatorial regions and mean latitudes.</p>

    <div class="sidenote">Polar Light
    due to electric
    discharges
    only.</div>

    <p>He sums up, that the electric discharges which take place in the Polar
    regions between the positive electricity of the atmosphere and the negative
    electricity of the earth are the essential and unique cause of the formation of
    the Polar Light, light the existence of which is independent of terrestrial magnetism,
    which contributes only to give to the Polar Light a certain direction,
    and in some cases to give it motion.</p>

    <p>This Prof. Lemström maintains contrary to those who believe they see in
    terrestrial magnetism, or rather in the induction currents, what is capable of
    developing the origin of the Polar Light.</p>

    <h4 id="chap-9-2"><i>Theories of MM. Becquerel and De la Rive.</i></h4>

    <div class="sidenote">Theories of
    MM. Becquerel
    and
    De la Rive.</div>

    <p>M. Becquerel’s theory is that solar spots are cavities by which hydrogen and
    other substances escape from the sun’s protosphere. That the hydrogen takes
    with it positive electricity which spreads into planetary space, even to the
    earth’s atmosphere and the earth itself, always diminishing in intensity because
    of the bad conducting-power of the successive layers of air and of the earth’s
    crust. That would then only be negative, as being less positive than the air.
    The diffusion of electricity through planetary space would be limited by the
    diffusion of matter, since it cannot spread in a vacuum. That gaseous matter
    extends further than the limits usually assigned to the earth’s atmosphere, is
    proved by the observation of Auroræ at heights of 100 and 200 kilometres,
    where some gaseous matter must exist. M. de la Rive agrees with
    M. Becquerel as to the electric origin of the Aurora, but considers the earth
    is charged with negative electricity and is the source of the positive atmospheric
    electricity, the atmosphere becoming charged by the aqueous vapour
    rising in tropical seas. The action of the sun he considers is an indirect one,
    varying with the state of the sun’s surface, as shown by coincidences in the
    periods of Aurora and sun-spots.</p>

    <p></p>

    <h4 id="chap-9-3"><i>M. Planté’s Electric Experiments.</i></h4>

    <div class="sidenote">M. Planté’s
    experiments. Effects produced
    resembling
    Auroræ.</div>

    <p>M. Planté has performed some experiments with a very considerable series
    of secondary batteries. By inserting the positive electrode after the negative
    in a vessel of salt water, luminous and other effects were observed which were
    considered to have a strong resemblance to those of Auroræ.</p>

    <p>M. Planté advocates the theory that the imperfect vacuum of the upper
    regions, acting like a large conductor, plays the part of the negative electrode
    in his experiments, while the positive electricity flows towards the planetary
    spaces, and not towards the ground, through the mists and ice-clouds which
    float above the Poles.</p>

    <div class="sidenote">M. Planté’s
    experiments
    producing
    a corona,
    an arc, or a
    sinuous line.</div>

    <p>In an article in ‘Nature,’ March 14, 1878, a further account is given of
    M. Planté’s experiments, under the head of “Polar Auroræ;” and it is stated
    that, in these experiments, the electric current, in presence of aqueous
    vapour, yielded a series of results altogether analogous to the various
    phases of Polar Auroræ. If the positive electrode of the secondary battery
    was brought into contact with the sides of a vessel of salt water, there was
    observed, according to the distance of the film (electrode?) from the liquid,
    either a corona formed of luminous particles arranged in a circle round the
    electrode (fig. 2, p. 90), an arc bordered with a fringe of brilliant rays (fig. 3), or
    a sinuous line which rapidly folded and refolded on itself (fig. 4). This undulatory
    movement, in particular, formed a complete analogy with what had been
    compared in Auroræ to the undulations of a serpent, or to those of drapery
    agitated by the wind. The rustling noise accompanying the experiment was
    analogous to that sometimes said to accompany Auroræ, and was caused by the
    luminous electric discharge penetrating the moisture. As in Auroræ, magnetic
    perturbations were produced by bringing a needle near the circuit, the
    deviation increasing with the development of the arch.</p>

    <p>The Auroræ were produced by positive electricity, the negative electrode
    producing nothing similar.</p>

    <p>Illustrations of these miniature Auroræ are given in ‘Nature,’ and reproduced
    on p. 90. No mention of any spectroscopic observations is made.</p>

    <div class="sidenote">Mr. Holden’s
    views.</div>

    <p>In a communication to the Metropolitan Scientific Association (‘Observatory,’
    March 1, 1879, p. 389), Mr. A. P. Holden, after supporting the theory
    of a connexion between the waxing and waning of the solar corona and sun-spots,
    adopts Mr. F. Pratt’s hypothesis “that the Aurora is simply light filmy
    cirrus cloud, first deposited at the base of a vast upper body of highly rarefied
    vapour, and illuminated by the free electricity escaping in the condensation
    through the very rarefied medium above, towards the north or south. The
    Aurora would, according to this theory, have its origin in a vast electrical
    storm, resulting from a violent condensation of vapour which causes a flow of
    electricity from the pole to restore equilibrium.” The Aurora would thus, in
    Mr. Holden’s opinion, “depend on storm phenomena of an intense character;
    and the frequency of Auroræ at the sun-spot maxima would indicate the connexion
    of the latter with the weather.”</p>

    <div class="figcenter" style="width: 700px;">
    <!-- <img src="assets/aurorae/images/fig2-3-4.jpg" width="700" height="375" alt="" /> -->
    <p class="caption">Fig. 2. The corona.</p>
    <p class="caption">Fig. 3. The arc and rays.</p>
    <p class="caption">Fig. 4. The sinuous line.</p>
    </div>

  </section>



  <!-- PART II_________________________________________________________ -->
  <section class="part">
    <h2  class="title-part"  id="part-2">The spectrum of the aurora</h2>
  </section>

  <!-- Chapitre 10_________________________________________________________-->
  <section class="chapter" id="chapter-10">
    <h3 class="titlechapter" id="chap-10">Spectroscope adapted for the Aurora</h3>
    <p class="shorter">Spectroscope adapted for the Aurora</p>

    <div class="sidenote">Must be of
    moderate
    dispersion,
    with ready
    mode of
    measuring
    line-positions.</div>

    <p>Any form of spectroscope of moderate dispersion will suffice for observations
    of the spectrum of the Aurora; but, for sake of convenience, a hand or
    direct-vision spectroscope is to be preferred, and it is desirable also to have
    some quick and ready mode of measuring the position of the lines while the
    Aurora lasts.</p>

    <div class="sidenote">Mr. Browning’s
    instrument
    described.</div>

    <p>Mr. John Browning arranged for me a form of instrument which I have
    found very convenient for observations by hand of the Aurora-lines, and also,
    when fixed on a stand, for tube and chemical investigations. A representation
    of this instrument is given on Plate X. fig. 1. A brass tube carries a large
    compound (5) direct-vision prism (shown dark in the drawing). An arrangement
    is made so that a second prism can at will be slipped into the tube
    (shown in outline in the drawing). With one prism and a fine slit the D
    lines are widely separated, and the field of view extends at one glance from
    near C to near G. When the second prism is inserted and used in combination,
    the nickel line can be seen between the two D lines, and the instrument
    may be used for solar work. A photograph of the sun’s spectrum, taken with
    one prism only, shows a great number of the dark solar lines and many of the
    bright ones, ascribed by Prof. Draper to oxygen and nitrogen.</p>

    <figure class="plate" id="plate10">
      <!-- <img src="assets/aurorae/images/plate10.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <p></p>

    <div class="sidenote">Diaphragm
    micrometer
    described. Mode of use
    of the
    micrometer.</div>

    <p>The collimator and observing telescope are respectively 6 inches in length,
    and carry achromatic lenses of one inch aperture. The telescope traverses
    the field so that the extremities of the spectrum may be observed. The
    dispersion of the instrument was ascertained by a set of observations of the
    principal solar and some metallic lines, made with an excellent filar micrometer.
    For the Auroral observations, Dr. Vogel has described an instrument
    (see Appendix E) in which the usual spider’s-web wires are replaced by a
    needle-point, as being easily seen upon a faint spectrum. Illuminated wires
    may also be used; but I was led ultimately to employ, in preference, a diaphragm
    micrometer which the spectrum itself illuminates, as being adapted
    for speedy, yet fairly accurate, observations. It was made in this manner:—A
    card was first of all prepared (Plate X. fig. 2), and within a circle described
    on this, a scale was drawn of moderately wide white spaces, with black
    divisions between, short and long, so as to read off easily by eye. The upper
    half of the circle was then entirely filled in with black; and from the card as
    thus prepared a reduced negative photograph was made. In this the spaces
    and lower half of the circle were opaque, and the upper half of the circle
    and the lines between the spaces were transparent (Plate X. fig. 4). This
    photograph was about the size of a shilling (fig. 3, same Plate). It was
    mounted carefully in Canada balsam, with a thin glass cover, and then
    placed in the focus of the eyepiece. In use, the spectrum is brought upon
    the scale so that the upper half shows above the scale without any interruption
    at all; while the lower half illuminates the scale and renders the
    divisions visible, showing the spectrum-lines falling either upon them or the
    spaces between. The photographed scale was next enlarged to a considerable
    size and printed upon faintly ruled paper; and the enlargement was so
    arranged as to comprise five of the faint ruled lines between each division of
    the scale. Each of these faint lines in turn represented a certain portion of
    the spectrum as read off with the filar micrometer; so that the scales as constructed
    with the filar micrometer and with the photographed micrometer
    corresponded for all parts of the spectrum included in the field of the eyepiece.</p>

    <div class="sidenote">Advantage
    of the
    method.</div>

    <p>One of the photographed enlargements being laid on the table under the
    spectroscope, the observed lines were marked off with ease and accuracy upon
    it; and as the photograph was an exact copy of the scale, any want of exactitude
    in the divisions was of no moment.</p>

    <p>One great advantage of this method was, that all the lines seen could be
    recorded at one time and with all in view, and without the risk of slight
    shift in the instrument, which frequently happens when lines are read off
    seriatim.</p>

    <p>I found this plan most effective for the rapid and correct recording of a
    faint and evanescent spectrum, and it gave close results when compared with
    traversing-micrometer measured spectra. The records, too, admitted of subsequent
    examination at leisure.</p>

    <div class="sidenote">Double-slit
    plate arrangement.</div>

    <p>Mr. Browning subsequently constructed for me a double-slit plate (lately in
    the Scientific Loan Collection at South Kensington) for the same instrument
    (Plate X. fig. 5). The lower half of the plate is fixed. The upper half
    traverses the lower by the aid of a micrometer-screw. The slit is widened or
    closed at pleasure by loosening the small screws by which the jaw-plates are
    attached. A scale is engraved on the fixed lower half of the plate for an
    approximate measurement, while the division of the micrometer-screw-head
    completes it.</p>

    <p>In use, one half of the spectrum slides along the other, and a bright line
    in the upper spectrum is selected as an index. The distances between the
    lines of the lower half of the spectrum are read off by means of the bright
    line above. This form of micrometer was suggested by Mr. Procter (in
    ‘Nature’) many years ago as a substitute for a more complicated apparatus
    by Zöllner. Other instruments on a similar principle have been lately introduced,
    but for Aurora purposes I prefer a fixed scale.</p>

    <div class="sidenote">Photographed
    spectrum
    suggested.</div>

    <p>In ‘Photographed Spectra’ I have pointed out that we shall probably
    obtain no spectrum of the Aurora to be absolutely depended upon for comparison
    with other spectra until we succeed in a photographed one. From
    experiments made with a special prism of the Rutherfurd form, constructed
    for me by Mr. Browning (with which many gas-spectra have been already
    photographed), I see no reason, should an unusually bright Aurora favour us
    with a visit, why its spectrum may not be recorded in a permanent form,
    and with lines sufficiently well marked to be compared with other spectra.
    Rapid dry plates would be especially useful for such a purpose, and some
    Auroræ, if wanting in brilliancy, would doubtless compensate by their period
    of endurance.</p>

    <div class="sidenote">Mr. Hilger’s
    half-prism
    spectroscope.</div>

    <p>Mr. Adam Hilger has also made for me one of his “half-prism” spectroscopes,
    in which considerable dispersion is obtained with but very little loss
    of light. This instrument has a simple and rapid micrometer arrangement,
    with a bright line as an index. I have (for want of Auroræ) had no opportunity
    of trying it, but I doubt not it is well adapted for such a purpose.</p>

    <p></p>

    <h4 id="chap-10-1"><i>Spectrum of the Aurora described.</i></h4>

    <div class="sidenote">Lines or
    bands and
    continuous
    spectrum.</div>

    <p>The spectrum of the Aurora consists of a set of lines or bands upon a dark
    ground at each extremity of the spectrum, but with more or less of faint continuous
    spectrum towards the centre. The extreme range of the spectrum,
    as observed up to the present time, is from “<i>a</i>” (between C and D) in the
    red to “<i>h</i>” (hydrogen) in the violet.</p>

    <div class="sidenote">Lines nine
    in number.</div>

    <p>The lines have been classified and arranged by Lemström and others as
    nine in number, but I believe not more than seven have ever been seen
    simultaneously.</p>

    <p>The author of the article “Aurora Polaris,” in the ‘Encyclopædia Britannica,’
    classes the lines as nine, and gives a table with the following results
    (to these I have added Herr Vogel’s lines, for the purpose of identification
    and comparison):—</p>

    <div class="sidenote">Table from
    Encyc. Brit.</div>

    <table summary="The nine classes of line" class="borders">
      <tr>
        <th>No. of<br />line.</th>
        <th>Number of<br />observations.</th>
        <th>Mean<br />W.L.</th>
        <th>Probable<br />error.</th>
        <th>Vogel’s<br />lines.</th>
      </tr>
      <tr>
        <td class="bl">1.</td>
        <td class="tdr">5</td>
        <td class="tdr">6303</td>
        <td>± 8·1</td>
        <td class="tdr">6297</td>
      </tr>
      <tr>
        <td class="bl">2.</td>
        <td class="tdr">10</td>
        <td class="tdr">5569</td>
        <td>± 2·9</td>
        <td class="tdr">5569</td>
      </tr>
      <tr>
        <td class="bl">3.</td>
        <td class="tdr">4</td>
        <td class="tdr">5342</td>
        <td>±16</td>
        <td class="tdr">5390</td>
      </tr>
      <tr>
        <td class="bl">4.</td>
        <td class="tdr">6</td>
        <td class="tdr">5214</td>
        <td>± 5·4</td>
        <td class="tdr">5233</td>
      </tr>
      <tr>
        <td class="bl">5.</td>
        <td class="tdr">4</td>
        <td class="tdr">5161</td>
        <td>± 9·7</td>
        <td class="tdr">5189</td>
      </tr>
      <tr>
        <td class="bl">6.</td>
        <td class="tdr">6</td>
        <td class="tdr">4984</td>
        <td>±11</td>
        <td class="tdr">5004</td>
      </tr>
      <tr>
        <td class="bl">7.</td>
        <td class="tdr">4</td>
        <td class="tdr">4823</td>
        <td>± 9·3</td>
        <td></td>
      </tr>
      <tr>
        <td class="bl">8.</td>
        <td class="tdr">8</td>
        <td class="tdr">4667</td>
        <td>± 9·8</td>
        <td class="tdr">4663</td>
      </tr>
      <tr>
        <td class="bl bb">9.</td>
        <td class="tdr bb">8</td>
        <td class="tdr bb">4299</td>
        <td class="bb">± 9·3</td>
        <td class="bb"></td>
      </tr>
    </table>

    <p>The probable errors are large, and it is a question whether any thing is
    gained by thus endeavouring to average the lines.</p>

    <div class="sidenote">Ångström’s
    line. Zöllner’s
    line in the
    red. Other Lines
    of the
    spectrum.</div>

    <p>The principal and brightest line, in the yellow-green, is generally called
    “Ångström’s,” and his (probably the first) measurement of its position at 5567
    adopted. This was in the winter of 1867-68, and he saw in addition, by
    widening the slit, traces of three very feeble bands situated near to F.
    Zöllner is credited with the first observation of the line in the red. These
    two lines are generally described as with similar characteristics, and in
    about the same respective positions, by all observers, and have never been
    remarked to spread into bands. The other lines in the spectrum are difficult
    to position, owing to the many discordant observations of them. They seem
    also variable in intensity as well as in number (sometimes even in the same
    Aurora), and are not unfrequently observed to have their places supplied by
    bands.</p>

    <div class="sidenote">Second
    German expedition
    observations. Austro-Hungarian.</div>

    <p>The spectroscope was used in the second German expedition, but only the
    one brightest line seen—Dr. Börgen stating he had never seen a trace of the
    weak lines in the blue and red, which were observed so distinctly with the
    same spectroscope on 25th October, 1870, after the return of the expedition.
    Lieutenant Weyprecht used a small spectroscope during the Austro-Hungarian
    Expedition, and saw only the well-known yellow-green line.</p>

    <div class="sidenote">Swedish expedition,
    1868. Lemström’s
    observations.</div>

    <p>In the Swedish Expedition, 1868, Lemström mentions that in the Aurora
    spectrum there are nine lines (he does not say he saw them simultaneously),
    which he considers to agree with lines belonging to the air-gases. He also
    thinks the Aurora could be referred to three distinct types, depending on the
    character of the discharge.</p>

    <div class="sidenote">Spectrum
    or Aurora
    seen at
    Tronsa.</div>

    <p>At Tronsa an Aurora was seen October 21st, 1868, which commenced in
    the north and became very brilliant. The spectroscope showed:—</p>

    <p>1. A yellow line at 74·9.</p>

    <p>2. A very clear line in the blue at 65·90.</p>

    <p>3. Two lines of hair’s breadth, with very pronounced (horizontal?) striæ on
    the side of the yellow, one at 125 and the other about 105.</p>

    <p>[I presume the striæ were really vertical, and that the explanation intended
    to convey that these lines shaded off towards the yellow. From a comparison
    of the figures they must have been in the red, and are the only instance
    recorded of two auroral lines in that region. They are subsequently spoken
    of as “shaded rays.”—J. R. C.]</p>

    <div class="sidenote">MM. Wijkander
    and
    Parent’s observations.</div>

    <p>M. Auguste Wijkander and Lieut. Parent, of the Swedish Expedition in
    1872-73, under Professor Nordenskiöld, used a direct-vision spectroscope,
    with a micrometer-screw movement of the prisms, the reading being afterwards
    reduced to wave-lengths upon Ångström’s line-values.</p>

    <p>The following Table gives the results, with Dr. Vogel’s lines added for the
    sake of comparison:—</p>

    <table summary="Results" class="borders">
      <tr>
        <th rowspan="2">Lines.</th>
        <th colspan="3">Observations, Wijkander.</th>
        <th colspan="3">Observations, Parent.</th>
        <th rowspan="2">Mean of<br />both.</th>
        <th rowspan="2">Vogel.</th>
      </tr>
      <tr>
        <th>Number.</th>
        <th>W.L.</th>
        <th>Probable<br />error.</th>
        <th>Number.</th>
        <th>W.L.</th>
        <th>Probable<br />error.</th>
      </tr>
      <tr>
        <td class="bl center">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">6297</td>
      </tr>
      <tr>
        <td class="bl center">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">5569</td>
      </tr>
      <tr>
        <td class="bl center">(1)</td>
        <td class="tdr">5</td>
        <td class="tdr">5359</td>
        <td class="tdr">±3</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">5359</td>
        <td class="tdr">5390</td>
      </tr>
      <tr>
        <td class="bl center">(2)</td>
        <td class="tdr">6</td>
        <td class="tdr">5289</td>
        <td class="tdr">±5</td>
        <td class="tdr">3</td>
        <td class="tdr">5280</td>
        <td class="tdr">± 1</td>
        <td class="tdr">5286</td>
        <td class="tdr">.. </td>
      </tr>
      <tr>
        <td class="bl center">(3)</td>
        <td class="tdr">6</td>
        <td class="tdr">5239</td>
        <td class="tdr">±4</td>
        <td class="tdr">2</td>
        <td class="tdr">5207</td>
        <td class="tdr">±11</td>
        <td class="tdr">5231</td>
        <td class="tdr">5233</td>
      </tr>
      <tr>
        <td class="bl center">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">5189</td>
      </tr>
      <tr>
        <td class="bl center">(4)</td>
        <td class="tdr">5</td>
        <td class="tdr">4996</td>
        <td class="tdr">±9</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">4996</td>
        <td class="tdr">5004</td>
      </tr>
      <tr>
        <td class="bl center">(5)</td>
        <td class="tdr">1</td>
        <td class="tdr">4871</td>
        <td class="tdr">..</td>
        <td class="tdr">1</td>
        <td class="tdr">4873</td>
        <td class="tdr">..</td>
        <td class="tdr">4872</td>
        <td class="tdr">.. </td>
      </tr>
      <tr>
        <td class="bl center">(6)</td>
        <td class="tdr">8</td>
        <td class="tdr">4692</td>
        <td class="tdr">±2</td>
        <td class="tdr">10</td>
        <td class="tdr">4708</td>
        <td class="tdr">± 5</td>
        <td class="tdr">4701</td>
        <td class="tdr">4663</td>
      </tr>
      <tr>
        <td class="bl center">(7)</td>
        <td class="tdr">1</td>
        <td class="tdr">4366</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">..</td>
        <td class="tdr">4366</td>
        <td class="tdr">.. </td>
      </tr>
      <tr>
        <td class="bl center bb">(8)</td>
        <td class="tdr bb">4</td>
        <td class="tdr bb">4280</td>
        <td class="tdr bb">±3</td>
        <td class="tdr bb">3</td>
        <td class="tdr bb">4286</td>
        <td class="tdr bb">±16</td>
        <td class="tdr bb">4284</td>
        <td class="tdr bb">..</td>
      </tr>
    </table>

    <p>The brightest line in all Auroræ, 5567, was intentionally not included in
    the Tables. The red line was not seen. Nos. 5 and 7 were only seen once,
    and not in the same Aurora.</p>

    <div class="sidenote">Spectrum
    of Aurora
    of October
    24th, 1870.</div>

    <p>The Aurora of October 24th, 1870, came at a time when spectroscopes of
    a direct-vision form were being introduced, and a number of observations were
    communicated at the time to ‘Nature.’</p>

    <div class="sidenote">T. F.’s observations. W. B.
    Gibbs’s observation. Elger’s observation.</div>

    <p>A correspondent, T. F., writing from Torquay, saw, with a direct-vision spectroscope,
    one strong red line near C, one strong pale yellow line near D, one
    paler near F, and a still paler one beyond, with a faint continuous spectrum
    from about D to beyond F. The C line was very conspicuous and the
    brightest of the whole. It was intermediate in position and colour to the red
    lines of the lithium and calcium spectra. Plainly there were two spectra
    superposed, for while the red portions of the Aurora showed the four lines
    with a faint continuous spectrum, the greenish portions showed only one line
    near D on a faint ground. W. B. Gibbs saw, in London, only two bright
    lines, one a greenish grey, situate about the middle of the spectrum, and the
    other a red line very much like C (hydrogen). Thomas G. Elger, at Bedford,
    on the 24th and 25th, saw:—(1) a broad and well-defined red band near C;
    (2) a bright white band near D (same as Ångström’s W.L. 5567), on 25th
    visible in every part of the sky; (3) a faint and rather nebulous line, roughly
    estimated to be near F; (4) a very faint line about halfway between 2 and 3.
    The red band was absent from the spectrum of the white rays of the Aurora,
    but the other lines were seen.</p>

    <div class="sidenote">J. R.
    Capron’s
    observation.</div>

    <p>With a small Browning direct-vision spectroscope on the 24th, I found no
    continuous spectrum, but two bright lines, one in the green (like that from
    the nebulæ, but more intense, and considerably flickering), the other in the
    red (like the lithium line, but rather duskier: Plate V. fig. 6). The latter
    was only well seen when the display was at its height; it could, however,
    be faintly traced wherever the rose tint of the Aurora extended. The line in
    the green was well seen in all parts of the sky, but was specially bright in
    the Auroral patches of white light.</p>

    <div class="sidenote">Mr. Browning’s
    observation. Alvan
    Clarke’s,
    jun., observations.</div>

    <p>Mr. Browning also saw the red line, but found comparison difficult. On
    the evening of the 24th October, Mr. Alvan Clarke, jun., at Boston, used a
    chemical spectroscope of the ordinary form, with one prism and a photographed
    scale illuminated with a lamp. Four Auroral lines were seen at
    points of his scale numbered 61, 68, 80, and 98. These were reduced to
    wave-lengths by Professor Pickering, with the following results:—</p>

    <table summary="Results" class="borders">
      <tr>
        <th>Line.</th>
        <th>Reading<br />on scale.</th>
        <th>Wave-lengths.</th>
        <th>Assumed<br />line.</th>
        <th>Comments.</th>
        <th>Probable<br />error.</th>
      </tr>
      <tr>
        <td class="bl center">1.</td>
        <td class="tdr">61</td>
        <td class="tdr">5690</td>
        <td class="tdr">5570</td>
        <td class="nw">Common Aurora-line.</td>
        <td class="tdr">-20</td>
      </tr>
      <tr>
        <td class="bl center">2.</td>
        <td class="tdr">68</td>
        <td class="tdr">5320</td>
        <td class="tdr">5316</td>
        <td>Corona line.</td>
        <td class="tdr">+ 1</td>
      </tr>
      <tr>
        <td class="bl center">3.</td>
        <td class="tdr">80</td>
        <td class="tdr">4850</td>
        <td class="tdr">4860</td>
        <td>F, hydrogen.</td>
        <td class="tdr">- 3</td>
      </tr>
      <tr>
        <td class="bl center bb">4.</td>
        <td class="tdr bb">98</td>
        <td class="tdr bb">4350</td>
        <td class="tdr bb">4340</td>
        <td class="bb">G, hydrogen.</td>
        <td class="tdr bb">+ 6</td>
      </tr>
    </table>

    <p>[61 is evidently wrong, and was probably a mistake for 63.]</p>

    <div class="sidenote">G. F. Barker’s
    observations.</div>

    <p>George F. Barker, observing at New Haven (U.S.A.), saw, on November
    9th, a crimson and white Aurora, which he examined with a single glass-prism
    spectroscope, by Duboscq, of Paris. The line positions were obtained
    by an illuminated millimetre scale. In the white Aurora were four lines
    (the red one being absent); in the red Aurora five. The wave-lengths of the
    Aurora-lines were run out as follows:—</p>

    <ul>
    <li>(1.) Between C and D, 6230 (Zöllner’s 6270).</li>
    <li>(2.) <span class="ditto2">”</span> D and E, 5620 (Ångström’s 5570).</li>
    <li>(3.) <span class="ditto2">”</span> E and <i>b</i>, 5170 (Winlock’s 5200).</li>
    <li>(4.) <span class="ditto2">”</span> <i>b</i> and F, 5020.</li>
    <li>(5.) <span class="ditto2">”</span> F and G, 4820 (Alvan Clarke’s, jun., 4850).</li>
    </ul>

    <p></p>

    <div class="sidenote">Spectrum
    of Aurora
    of Feb. 4,
    1872. Prof. Piazzi
    Smyth’s observations.</div>

    <p>Mr. Procter’s Aurora-lines will be found noticed in connexion with the
    spectrum of oxygen; and Lord Lindsay’s lines, with a comparison scale
    drawing, are separately described further on in this Chapter. The Aurora of
    February 4th, 1872, had many observers; some of whom communicated at
    the time spectroscopic notes. Professor Piazzi Smyth minutely describes the
    display as seen in Edinburgh, and saw “Ångström’s green Aurora-line perpetually
    over citron acetylene&nbsp;<span class="footnote">There seems to be some confusion as to the W.L. here given; 5567 is usually accepted as
  Ångström’s line, while Prof. Smyth refers to it as 5579. The position, too, when examined with
  a spectroscope of greater dispersion, is not exactly over the citron-line of acetylene, both the above
  referred to lines lying somewhat more towards the violet end of the spectrum (see Plate V. fig. 7).</span> at W.L. 5579, and the red Aurora-line
    between lithium <i>a</i> and sodium <i>a</i>, but nearer to the latter, say at W.L. 6370.”
    Extremely faint greenish and bluish lines also appeared at W.L. 5300, 5100,
    and 4900 nearly.</p>

    <div class="sidenote">Rev. T. W.
    Webb’s observations.</div>

    <p>The Rev. T. W. Webb, with a very fine slit, saw the green Auroral line even
    in the light reflected from white paper. With a wider slit he saw a crimson
    band in the brighter patches of that hue, and beyond an extent of greenish
    or bluish light, which he suspected to be composed of contiguous bands.</p>

    <div class="sidenote">R. J. Friswell’s
    observations.</div>

    <p>R. J. Friswell, coming up the Channel at 9.40, with a Hoffman’s direct-vision
    spectroscope (the observing telescope removed), saw the green line, a
    crimson line near C, and faint traces of structure in the blue and violet.</p>

    <div class="sidenote">The Rev.
    S. J. Perry’s
    observations.</div>

    <p>The Rev. S. J. Perry observed at Stonyhurst four lines, and, on examining
    one of the curved streamers, found the red line even more strongly marked
    than the green. A magnetic storm was observed to be at its height from 4
    to 9 <span class="smcapuc">P.M.</span> of the same day.</p>

    <div class="sidenote">J. R. Capron’s
    observations.</div>

    <p>With a Browning 7-prism direct-vision spectroscope I saw the green line
    in all parts of the Aurora, attended with a peculiar flickering movement. I
    did not see the other lines.</p>

    <div class="sidenote">His catalogue
    of
    lines up to
    Nov. 9,
    1872.</div>

    <p>In a letter to ‘Nature,’ dated November 9th, 1872, I catalogued the lines
    observed up to that date as follows:—</p>

    <p>1. A line in the red between C and D. W.L., Ångström, 6279.</p>

    <p>2. A line (the principal one of the Aurora) in the yellow-green, between
    D and E. W.L., Ångström, 5567.</p>

    <p>3. A line in the green, near E (corona line?). W.L., Alvan Clarke, jun.,
    and Backhouse, 5320.</p>

    <p>4. A faint line in the green, at or near <i>b</i>. W.L., Barker, 5170.</p>

    <p>5. A faint line or band in the green, between <i>b</i> and F. W.L., Barker,
    5020 (chromospheric?).</p>

    <p></p>

    <p>6. A line in the green-blue, at or near F. W.L., Alvan Clarke, jun.,
    4850.</p>

    <p>7. A line in the indigo, at or near G. W.L., Alvan Clarke, jun., 4350.</p>

    <p>8. The continuous spectrum from about D to beyond F.</p>

    <div class="sidenote">Dr. H.C.
    Vogel’s observations
    of Auroral
    lines. Spectrum
    described.</div>

    <p>Dr. H. C. Vogel, formerly of the Bothkamp Observatory, near Kiel, and
    since of the Astrophysical Observatory, Potsdam, made several observations
    of the Auroral lines, October 25th, 1870. Besides the bright line between
    D and E, he found several other fainter lines stretching towards the blue
    end of the spectrum on a dimly-lighted ground. February 11th, 1871, he
    observed the same set of lines, and an average of six readings gave 5572 as
    the W.L. of the Ångström line. February 12th gave 5576 as Dr. Vogel’s
    reading, and 5569 as Dr. Lohse’s. April 9th gave 5569, and April 14th
    5569. The Aurora of April 9th, 1871, was exceedingly brilliant, so that
    micrometer measurements of the lines were taken. The spectrum consisted
    of one line in the red, five in the green, and a somewhat indistinct broad line
    or band in the blue. The lines are thus described:—</p>

    <div class="sidenote">Table of
    lines.</div>

    <p class="center">Table of Dr. Vogel’s lines. Aurora, April 9th, 1871.</p>

    <table summary="Dr. Vogel’s lines" class="borders">
      <tr>
        <th>W.L.</th>
        <th>Probable<br />error.</th>
        <th colspan="2">Remarks.</th>
      </tr>
      <tr>
        <td class="bl tdr">6297</td>
        <td class="tdr">14</td>
        <td>Very bright stripe.</td>
        <td rowspan="3" class="vm">&nbsp;On a faintly lighted ground.</td>
      </tr>
      <tr>
        <td class="bl tdr">5569</td>
        <td class="tdr">2</td>
        <td>Brightest line of the spectrum, became noticeably fainter at appearance of the red line.</td>
      </tr>
      <tr>
        <td class="bl tdr">5390</td>
        <td class="tdr">..</td>
        <td>Extremely faint line; unreliable observation.</td>
      </tr>
      <tr class="spaced">
        <td class="bl tdr">5233</td>
        <td class="tdr">4</td>
        <td colspan="2">Moderately bright.</td>
      </tr>
      <tr class="spaced">
        <td class="bl tdr">5189</td>
        <td class="tdr">9</td>
        <td colspan="2">This line was very bright when the red line appeared at the same time; otherwise equal in brilliancy with the preceding one.</td>
      </tr>
      <tr class="spaced">
        <td class="bl tdr">5004</td>
        <td class="tdr">3</td>
        <td colspan="2">Very bright line.</td>
      </tr>
      <tr class="spaced">
        <td class="bl tdr">4694</td>
        <td rowspan="3" class="tdr bb">3</td>
        <td rowspan="3" colspan="2" class="bb">Broad band of light, somewhat less brilliant in the middle; very faint in those parts of the Aurora in which the red line appeared.</td>
      </tr>
      <tr>
        <td class="bl tdr">4663</td>
      </tr>
      <tr>
        <td class="bl tdr bb">4629</td>
      </tr>
    </table>

    <p>A translation of Dr. Vogel’s interesting paper will be found printed <i>in extenso</i>
    in Appendix E, and his lithographed drawings of the spectrum in the
    green and red portions of the Aurora respectively on Plate VI. figs. 2 and 3.
    The observations of April 9th by Dr. Vogel are probably, up to the present
    time, the most exact of any one Aurora, and I have therefore in most cases
    used them for comparison.</p>

    <div class="sidenote">Mr. Backhouse’s
    catalogue of
    lines.</div>

    <p>Mr. Backhouse, in a letter to ‘Nature,’ commenting upon my catalogue
    of lines, gave the following as the latest determinations from his own
    observations:—</p>

    <table summary="Backhouse's observations">
      <tr>
        <td class="tdr">No. 1.</td>
        <td>Wave-length</td>
        <td class="tdr">6060</td>
      </tr>
      <tr>
        <td class="tdr">2.</td>
        <td class="center">”</td>
        <td class="tdr">5660</td>
      </tr>
      <tr>
        <td class="tdr">3.</td>
        <td class="center">”</td>
        <td class="tdr">5165</td>
      </tr>
      <tr>
        <td class="tdr">4.</td>
        <td class="center">”</td>
        <td class="tdr">5015</td>
      </tr>
      <tr>
        <td class="tdr">6.</td>
        <td class="center">”</td>
        <td class="tdr">4625</td>
      </tr>
      <tr>
        <td class="tdr">7.</td>
        <td class="center">”</td>
        <td class="tdr">4305</td>
      </tr>
    </table>

    <p class="noindent">(6060 must be a mistake for 6260, and 5660 for 5560.—J. R. C.) Mr. Backhouse
    never saw a line at 5320 again. He found the continuous spectrum to
    reach from No. 2 to No. 7, being brightest from a little beyond No. 2 to
    No. 6. This part of the spectrum did not give him so much the idea of
    a true “continuous spectrum” as of a series of bright bands too close to be
    distinguished.</p>

    <div class="sidenote">Subsequent
    full catalogue
    of Auroral
    lines.</div>

    <p>I have subsequently, in another section of this Chapter, added a full catalogue
    of the Auroral lines, prepared by myself from the foregoing and other
    sources and observations; and I also append to it a Plate [Plate XII.], in
    which these lines are positioned and the wave-lengths and names of observers
    are given. The numbers of the lines on the Plate correspond with those in
    the catalogue. The solar spectrum and the spectrum of the blue base of a
    candle-flame are added for purposes of comparison. [The telluric bands in
    the solar spectrum are shown more distinctly than they actually appear, and
    do not profess to give details.]</p>

    <h4 id="chap-10-2"><i>Flickering of the Green Line.</i></h4>

    <div class="sidenote">Flickering of
    the green
    line. Herschel’s
    observation. J. R. Capron’s
    observation.</div>

    <p>A. S. Herschel noticed this, April 9th (1871?). He says:—“A remarkable
    circumstance connected with the appearance of the single line observed on
    this occasion was the flickering and frequent changes with which it rose and
    fell in brightness; apparently even more rapidly than the swiftly travelling
    waves, or pulsations of light, that repeatedly passed over the streamers, near
    the northern horizon, towards which the spectroscope was directed.” In the
    spectrum of the Aurora of 20th October, 1870, I saw and noted the green
    line as “considerably flickering;” and in the Aurora of 4th February, 1872,
    I again saw and noted “the peculiar flickering” I had remarked in 1870. I
    have not seen the peculiarity noted by other observers.</p>

    <h4 id="chap-10-3"><i>Mr. Backhouse’s graphical Spectra of four Auroræ.</i></h4>

    <div class="sidenote">Mr. Backhouse’s
    graphical
    spectra of
    Auroræ.</div>

    <p>Mr. Backhouse has been good enough to supply me with some details of
    four several Auroræ seen by him at Sunderland, accompanied by drawings,
    showing in a graphical way the spectrum of each display as seen with a spectroscope
    with rather a wide slit and as drawn by eye. I have reduced the four
    drawings to the same scale, and in this way they are extremely interesting for
    comparison (Plate V. fig. 4). The line on the left in each spectrum is Ångström’s
    bright Auroral line, and is supposed to be considerably prolonged.
    The height of the lines denotes intensity.</p>

    <div class="sidenote">April 18,
    1873.</div>

    <p>April 18th, 1873, was a bright Aurora. No. 3 is a faint band, which
    Mr. Backhouse had not perceived before. No. 5 had not been visible lately,
    and Mr. Backhouse thought it must belong to Auroræ of a different type
    from those which had appeared latterly.</p>

    <div class="sidenote">Feb. 4,
    1874.</div>

    <p>February 4th, 1874. In the spectrum of this Aurora Mr. Backhouse saw
    seven lines, all that he had ever seen. (The red line, not shown in the diagram,
    makes the seventh.)</p>

    <p>The spectrum is represented as seen between 6.50 and 7.5 <span class="smcapuc">P.M.</span> Mr. Backhouse
    had only once before seen No. 4, and it became quite invisible between
    7.45 and 7.55, though the other lines were as bright as before and the red
    line had appeared.</p>

    <span class="sidenote">Oct. 3, 1874.</span>

    <p>October 3rd, 1874. This spectrum was examined, and diagram made between
    10 and 10.25 <span class="smcapuc">P.M.</span> Five lines only are indicated.</p>

    <p>It is mainly distinguished from the two preceding spectra by the brightness
    of the continuous spectrum on which the lines 2, 3, and 4 lie, and by the
    weakness of No. 5.</p>

    <span class="sidenote">Oct. 4, 1874.</span>

    <p>October 4th, 1874. Taken between 11.10 and 11.20 <span class="smcapuc">P.M.</span>; distinguished,
    like the last, by a considerable amount of continuous spectrum and by a faint
    line (No. 3), not seen in the last spectrum, while No. 3 in the last is missing
    in this spectrum.</p>

    <div class="sidenote">Mr. Backhouse’s
    remarks
    as to
    comparative
    frequency of
    some of the
    Auroral
    lines.</div>

    <p>Mr. Backhouse, as to both these last spectra, remarks that the lines were
    very variable in intensity, and sometimes some were visible and sometimes
    others. They varied also in relative brightness in different parts of the sky
    at the same time. Mr. Backhouse, in a communication to ‘Nature,’ referring
    to a statement of Mr. Procter’s, that the bands of the Auroral spectrum are
    seldom visible, except the bright line at 5570, says that he always found two
    bands, “doubtless Winlock’s 4640 and 4310,” to be invariably visible when
    the Aurora was bright enough to show them. Of thirty-four Auroræ examined
    by Mr. Backhouse, fourteen showed the lines 4640 and 4310, and
    three others at least one of these, while eight showed the red line. (Ångström
    only once saw this line.) In five Auroræ, all more or less red, he saw
    a faint band, the wave-length of which he placed at 5000 or 5100. He never
    saw the line 5320 (also Winlock’s coronal line), unless it were once, probably
    from want of instrumental power. With regard to these observations, I may
    say that with a Browning’s miniature spectroscope I saw only two lines (the
    red and the green) in the grand display of the 24th October, 1870; and with
    an instrument of larger aperture the green line only on the 4th February,
    1872; while I saw the green line and three others towards the violet with
    the same instrument during the Aurora of 4th February, 1874. (See description
    of this Aurora, <i>antè</i> p. 21, and drawing of spectrum, Plate VI. fig. 1 <i>a</i>.)</p>

    <h4 id="chap-10-4"><i>Lord Lindsay’s Aurora-Spectrum, 21st October, 1870.</i></h4>

    <div class="sidenote">Lord Lindsay’s
    Aurora
    of 21st Oct.,
    1870.</div>

    <p>Lord Lindsay observed a fine Aurora at the Observatory at Dun Echt on
    the night of the 21st October, 1870. It commenced about 9.30, reached its
    maximum about 11, and faded away suddenly about 11.30 <span class="smcapuc">P.M.</span></p>

    <div class="sidenote">Spectrum
    described.</div>

    <p>A spectrum obtained in the north-west gave five bright lines with a Browning’s
    direct-vision spectroscope—two strong, one medium, two very faint. A
    tallow candle was used to obtain a comparison spectrum of sodium and carburetted
    hydrogen.</p>

    <p>A drawing of the spectrum obtained is given on Plate XI. fig. 2. No. 1
    is a sharp well-formed line visible with a narrow slit.</p>

    <p>No. 2, a line very slightly more refrangible than F. The side towards D
    is sharp and well defined, while on the other side it is nebulous.</p>

    <p>No. 3, slightly less refrangible than G, is a broad ill-defined band, seen only
    with a wide slit.</p>

    <p>No. 4, a line near E, woolly at the edges, but rather sharp in the centre.
    This, says Lord Lindsay, should be at or near the position of the line 1474 of
    the solar corona.</p>

    <p>No. 5, a faint band, coincident with <i>b</i>, extending equally on both sides
    of it.</p>

    <p>The lines are numbered in order of intensity. It is questionable, from
    observations with instruments carrying a scale, whether the line-positions are
    exact; but the description of their characters is valuable.</p>

    <p></p>

    <span class="sidenote">Candle-spectrum.</span>

    <p>As a candle blue-base spectrum is at times a ready and handy mode of
    reference in Auroral observations (as was found in this instance), I have, on
    Plate XI. fig. 5, given a representation of it as seen with my Auroral spectroscope.
    Dr. Watts’s corresponding carbon-spectrum is added on the lower
    margin. The numbers on the upper margin refer to my scale.</p>

    <figure class="plate" id="plate11">
      <!-- <img src="assets/aurorae/images/plate11.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-10-5"><i>Spectrum of the Aurora Australis.</i></h4>

    <div class="sidenote">Captain
    Maclear’s
    spectra of
    Aurora Australis.</div>

    <p>Captain Maclear, on examining the streamer seen by him Feb. 9th, 1874
    (<i>antè</i>, p. 27), with the spectroscope, found three prominent lines in the yellow-green,
    green, and blue or purple, but not the red line. In the Aurora of
    March 3rd, 1874 (p. 27), he could trace four lines, three bright and one rather
    faint. They must have been exceedingly bright to show so plainly in full-moon
    light.</p>

    <div class="sidenote">Instrument
    used, and
    mode of
    registering
    lines.</div>

    <p>The spectroscope used was a Grubb single-prism with long collimator.
    A needle-point in the eyepiece marked the position of the lines; and a corresponding
    needle-point, carried on a frame by a screw movement in concord
    with the point of the eyepiece, scratched the lines on a plate of blackened
    glass. Two plates were taken. On the first were scratched the auroral
    lines and the solar lines as seen in the moonlight; on the second plate were
    scratched the auroral lines, the Solar lines from the moon, and the carbon
    lines in a spirit-lamp.</p>

    <div class="sidenote">Copies of
    the two
    spectra obtained. Discrepancy
    in the spectra. Remarks on
    the spectra.</div>

    <p>The next morning the solar lines were verified in sunlight. I subjoin
    (Plate XI. fig. 3) copies of the two spectra as printed in ‘Nature,’ the auroral
    lines being marked A, the solar lines by the usual designating letters, and
    the carbon by <i>Car</i>. To these spectra I have added for comparison Dr. Vogel’s
    spectrum of the Aurora Borealis. Captain Maclear could not account for the
    different positions of the auroral lines in the two plates; for the prism, as far
    as he was aware, was not moved during the observations. As the solar lines
    are indicated in the same place in both spectra, the case would seem one of
    actual change of position of the auroral lines during observation. A comparison
    of the two spectra gives the impression that the lower one is the same
    as the upper, except that the dispersion is greater, the lines remaining relatively
    in position. One does not, however, see how the dispersion could have
    so varied in a single-prism instrument, and the position of the solar lines is
    adverse to such an explanation.</p>

    <p>There is a suspicion that Auroræ are not always identical in position of
    some of the lines; but the line in the green (considerably out of place in the
    lower Australis spectrum) has always, within small limits, the same position.
    It will be noticed how much further the Australis spectrum runs into the
    violet than Vogel’s Borealis, the latter having no lines much beyond F.</p>

    <p>The faint line (No. 2) mentioned by Captain Maclear possibly corresponds
    with Dr. Vogel’s band. The absence of the four lines of the Aurora Borealis
    in the green part of the spectrum of the Australis is peculiar; and in this
    respect, too, the two Australis spectra agree.</p>

    <div class="sidenote">Comparison
    of the lines.</div>

    <p>The nearest approaches to Captain Maclear’s lines (of the upper spectrum)
    which I can find are:—</p>

    <table summary="The nearest approaches to Captain Maclear’s lines (of the upper spectrum)">
      <tr>
        <th>Line</th>
        <th>Corresponding line.</th>
      </tr>
      <tr>
        <td class="tdr">1.</td>
        <td>5567, Ångström.</td>
      </tr>
      <tr>
        <td class="tdr">2.</td>
        <td>(The faint line.) Vogel’s band, 4694-4629.</td>
      </tr>
      <tr>
        <td class="tdr">3.</td>
        <td>I find no approximately corresponding line.</td>
      </tr>
      <tr>
        <td class="tdr">4.</td>
        <td>4350, Alvan Clarke.</td>
      </tr>
    </table>

    <p class="noindent">But the comparisons are not by any means close. Further observations of the
    Australis spectrum are very desirable.</p>

    <h4 id="chap-10-6"><i>Prof. Piazzi Smyth’s Aurora-Spectra.</i></h4>

    <div class="sidenote">Prof. Piazzi
    Smyth’s
    chemical
    and auroral
    spectra.</div>

    <p>Prof. Piazzi Smyth, in volume xiv. of the ‘Edinburgh Astronomical Observations,’
    1870-77, has compared simultaneously the Aurora-spectrum with
    the sets of bright lines seen in the blue base of flame—the lines of potassium,
    lithium, sodium, thallium, and indium being also introduced for comparison.
    The spectra are drawn as seen under small dispersion, and will prove most
    useful in cases where an Aurora is not bright enough to admit of the lines
    being measured by micrometer, and the eye and comparison spectrum are
    obliged to be resorted to.</p>

    <figure class="plate" id="plate12">
      <!-- <img src="assets/aurorae/images/plate12.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-10-7"><i>Author’s Catalogue of the Auroral Lines.</i></h4>

    <p class="center">(See Plate XII.)</p>

    <p>1. W.L. 6297, Vogel. Very bright stripe; first noticed by Zöllner. Seen
    only in red Auroræ; stands out on a dark ground, without other lines near it.
    Character of line sharp and well defined; varies in colour from dusky red to
    bright crimson. Intensity, Herschel, 0 to 4 or 8. According to same, position
    coincident with atmospheric absorption-group “<i>a</i>” in solar spectrum (between
    C and D). I confirm this position according to my scale of solar lines, and a
    drawing of the coincidence (in which, and in Plate XII., the absorption-lines
    are drawn too dark) is given on Plate XIII. fig. 2. Herschel says this line
    coincides with a red band in the negative glow-discharge, but its identity is
    doubtful. Its isolation and want of adjacent lines seem to separate it from
    the air-spectrum and gas-spectra in general. At the appearance of this line,
    5569 (No. 2) becomes noticeably fainter. When this line is bright, 5189
    (No. 5) is bright also (Vogel).</p>

    <p>If we propose to assign to this line, as well as to 5569, a phosphorescent
    origin, it would be strongly confirmatory of such a theory (in connexion with
    the phosphoretted-hydrogen spectrum) to find it brighten at low temperatures.</p>

    <p><i>Note.</i>—Sir John Franklin says, in his ‘Polar Expeditions,’ that a low state of
    temperature is favourable for the production of brilliant coruscations. It was
    seldom witnessed that the Auroræ were much agitated, or that the prismatic
    tints were very apparent, when the temperature was above zero.</p>

    <p>2. Line in the yellow-green. Brightest of all lines in the Aurora-spectrum.
    W.L. 5567, Ångström; 5569, Vogel. Intensity 25, Herschel. To me more
    pale green than yellow, sometimes flickering and changing in brightness
    (Herschel and Capron). Seen in all Auroræ usually sharp and bright, but
    Procter has once recorded it nebulous. Its character as to width, sharpness,
    and intensity, if carefully observed, might indicate height and structure of
    Aurora. Becomes noticeably fainter at appearance of red line (Vogel).
    Found by me to correspond in position with a faint atmospheric absorption-band
    (see Plate XIII. fig. 2). According to Ångström and Herschel, arising
    from a phosphorescent and fluorescent light, emitted when air is subjected to
    the action of electrical discharge.</p>

    <p>3. Line in green near last. W.L. 5390. An extremely faint and unreliable
    observation (Vogel). Seen only by him, unless Alvan Clarke’s 5320 (coronal?)
    be the same.</p>

    <p>4. Line in green-blue. W.L. 5233, moderately bright (Vogel); 5200, Winlock.
    Intensity, 2 or 0? to 6, Herschel. Coincides with line in the negative
    glow according to same. Frequently observed.</p>

    <p>5. Line in green-blue. W.L. 5189. This line is very bright when the red
    line appears at the same time; otherwise equal in brilliancy with No. 3 (Vogel);
    Winlock, 5200. Not so frequently observed as No. 3. Barker gives a band
    extending from 5330 to 5200. Intensity of 5189, 0 to 8, Herschel, who considers
    it coincident with a constant strong line in the spark-discharge.</p>

    <p>6. Line in blue. W.L. 5004. Very bright line, Vogel; 5020, Barker
    (coronal?). Intensity 2 or 0? to 8, Herschel. Coincides with line of nitrogen
    in the nebulæ according to same. Barker gives a band extending from 5050
    to 4990.</p>

    <p>7. Line in the blue not found by Vogel in Aurora, April 9th, 1871. W.L.
    4850, Alvan Clarke; 4820, Backhouse and Barker. Intensity, Herschel, of
    4820-4870, 0 to 4? Herschel suspects this and No. 4 to be seen only in
    Auroral streamers of low elevation. Barker gives a band extending from
    4930 to 4850.</p>

    <p>8. 4694, 4663, 4629. Broad band of light, somewhat less bright in the
    middle; very faint in those parts of the Aurora in which the red line appears
    (Vogel). Intensity 3-6 (Herschel). A double band, consisting of two lines,
    the first rather more frequently noted than the second in Auroral spectra,
    agrees well in position with the principal band in the negative glow-spectrum
    (same). Barker gives a band extending from 4740 to 4670; Backhouse and
    Winlock give a line at 4640, situate within the same.</p>

    <p>9. There seems a good deal of confusion about a fairly bright line (intensity
    0-6, Herschel) seen in most Auroræ (not, however, by Vogel, April 9th, 1871),
    and situate somewhere near G in the solar spectrum. Alvan Clarke places it
    at 4350, on the less refrangible side of G; Backhouse and Barker at or very
    near to G; while Lemström and others position it on the more refrangible
    side of G. Accurate observations, for which a quartz spectroscope might be
    useful, are much wanted. Herschel makes this line, at 4285, correspond with
    a strong band in the violet in the negative glow-spectrum.</p>

    <p>Herschel also refers to an apparently additional line near the hydrogen-line,
    or between G and H₁, in the solar spectrum, as mentioned once by Lemström
    at Helsingfors. I am not aware of any other observation of this line,
    which must be considerably beyond that at or near G, and would probably
    be difficult to detect, except in instruments specially adapted for examination
    of the violet end of the spectrum.</p>

    <h4 id="chap-10-8"><i>Theories in relation to the Aurora and its Spectrum.</i></h4>

    <span class="sidenote">Lemström’s.</span>

    <p>Lemström (1):—That the Polar light is caused by an electric current passing
    from the upper rarefied layers of the air to the earth, producing light-phenomena
    that do not arise in the denser layers of the air. (2) That there are
    nine rays (lines or bands) in the Aurora-spectrum, which in all probability
    agree with lines which belong to the gases of the air. (3) That the Aurora-spectrum
    can be referred to three distinct types, which depend on the
    character of the discharge.</p>

    <p></p>

    <span class="sidenote">Vogel’s.</span>

    <p>Dr. Vogel:—(1) That the Auroræ are electric discharges in rarefied-air strata
    of very considerable thickness. (2) That the Aurora-spectrum is a modification
    of the air-spectrum, involving the question of alteration of the spectrum
    by conditions of temperature and pressure.</p>

    <span class="sidenote">Ångström’s.</span>

    <p>Ångström:—(1) seems to adopt the hypothesis that the Aurora has its final
    cause in electrical discharges in the upper strata of the atmosphere, and that
    these, whether disruptional or continuous, take place sometimes on the outer
    boundary of the atmosphere, and sometimes near the surface of the earth.</p>

    <p>(2) That the Aurora has two different spectra.</p>

    <p>(3) That the green line is due to fluorescence or phosphorescence, and that
    there is no need to resort to Dr. Vogel’s variability of gas-spectra according
    to circumstances of pressure and temperature.</p>

    <p>(4) That an agreement exists between the lines of the Aurora (except the
    red and green before mentioned) and the lines or bands of the violet light
    which proceed from the negative pole in dry air.</p>

    <div class="sidenote">Zöllner’s
    remark as to
    temperature
    of Aurora
    and character
    of spectrum.</div>

    <p>Zöllner has pointed out that the temperature of the incandescent gas of the
    Aurora must be exceedingly low, comparatively, and concludes that the
    spectrum does not correspond with any known spectrum of the atmospheric
    gases—only because, though a spectrum of our atmosphere, it is one of
    another order, and one which we cannot produce artificially.</p>

  </section>

  <!-- Chapitre 11_________________________________________________________-->
  <section class="chapter" id="chapter-11">
    <h3 class="titlechapter" id="chap-11">The comparison of some Tube and other Spectra with the Spectrum of the Aurora</h3>
    <p class="shorter">The comparison of some Tube and other Spectra</p>

    <p class="center">[In part from an Article in the ‘Philosophical Magazine’ for April 1875.]</p>

    <div class="sidenote">Testing
    Ångström’s
    Aurora
    theory. Battery and
    spectroscope
    described. Vogel’s spectrum
    selected
    for
    comparison.</div>

    <p>In order to test Professor Ångström’s theory of the Aurora, referred to in the
    last Chapter, in an experimental way, I examined, in the winter of 1874, some
    tube and other spectra, not only for line-positions, but also for general resemblance
    to an Aurora-spectrum. It did not seem desirable to use powerful
    currents. A ½-inch-spark coil, worked by a quart bichromate-cell, was found
    sufficient to illuminate the tubes steadily. The spectroscope used was one
    made for me by Mr. Browning specifically for Auroral purposes, and of the
    direct-vision form, being the same instrument as is described <i>antè</i>, p. 91, and
    figured in Plate X. fig. 1. The micrometer was the diaphragm one, also before
    described and figured on same Plate, figs. 2, 3, and 4. I selected Dr. Vogel’s
    spectrum for comparison, it being, so far as I am aware, the most accurately
    mapped, with regard to wave-length, at one observation, of any Auroral spectrum.
    It seemed an unsafe plan to attempt to obtain an average Aurora by
    comparison of different observations made at various times by different observers
    with all sorts of instruments—the difficulty, too, being increased by
    the suspicion that the spectrum itself at times varies in number and position
    as well as intensity of its lines.</p>

    <div class="sidenote">Central part
    only of spectrum
    mapped.</div>

    <p>In most cases the central part of the spectrum only (corresponding to the
    central lines of the Aurora) was mapped, the red line in the Aurora not being
    found to correspond with any prominent line in the gas-spectra examined,
    and the Auroral line near solar G being so indefinitely fixed as to render
    comparison almost valueless. (See Plate XIII. fig. 1.)</p>

    <p>Dr. Vogel’s spectrum does not comprise the line near G; but I have added
    this (in an approximate place only) in order to complete the set of lines. For
    drawing of Dr. Vogel’s spectrum, with its scales attached, see Plate XIII.</p>

    <figure class="plate" id="plate13">
      <!-- <img src="assets/aurorae/images/plate13.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-11-1"><i>Hydrogen-tube.</i></h4>

    <div class="sidenote">Hydrogen-tube. Colour of
    glow varied
    with intensity
    of current.</div>

    <p>This tube was one of Geissler’s and of rather small calibre. On illumination
    the wide ends were easily lighted with a silver-grey glow, having a
    considerable amount of stratification. The capillary part glowed brilliantly
    with silver-white, bright green, and crimson light, according to the intensity of
    the current. With the commutator slowly working, white running into green
    and bright green were the main features of the thread of light; on the
    current passing more rapidly, the capillary thread became of an intense
    crimson, at the same time apparently increasing in diameter (an effect probably
    due to irradiation).</p>

    <p></p>

    <div class="sidenote">Spectrum
    described.</div>

    <p>The spectrum was very brilliant, consisting of the three bright lines
    usually distinguished as Hα, Hβ, and Hγ, and a number of shaded bands
    and fainter lines between these, with a bright continuous spectrum as a background
    to the whole.</p>

    <div class="sidenote">Lines α, β,
    and γ varied
    in intensity
    with colour
    as seen by
    eye. Fainter lines
    or bands described.</div>

    <p>The lines Hα, Hβ, and Hγ were found to vary in intensity with the
    current, and in accordance with the colour of the light as seen by the eye—a
    fact, as I think, not without bearing on the question of the Aurora, the varying
    tints of which are so well known. The fainter lines or bands were mostly
    stripes of pretty equal intensity throughout, and all about the width of the
    Hβ line. I did not trace any marked degrading on either side of the lines,
    though the edges were not uniformly so sharp as Hα and Hβ. Some of the
    lines were found coincident in position with lines of the air-spectrum.</p>

    <div class="sidenote">Purity of
    subsidiary
    lines questioned.</div>

    <p>It is a question whether these subsidiary lines are hydrogen, or are due to
    some tube impurity. A photograph I have taken of this tube-spectrum
    shows 17 lines in the part of the spectrum between F and H₂, some of which
    are repeated in the hydrocarbon-tube spectra.</p>

    <div class="sidenote">Coincidence
    of lines with
    Aurora-spectrum.</div>

    <p>No principal line, and one subsidiary line only, actually coincide with the
    Aurora-spectrum, this last being that to which Dr. Vogel assigns an identical
    wave-length, viz. 5189. Other of the subsidiary lines, however, fall somewhat
    near the Aurora-lines 5569, 5390, 5233, and 5004, two faint lines also
    falling within the band 4694 to 4629.</p>

    <div class="sidenote">Comparison
    of the lines.</div>

    <p>The lines (adopting Dr. Vogel’s wave-lengths for the H lines) were, when
    compared, as under:—</p>

    <table summary="Comparison of the lines">
      <tr>
        <td>Aurora</td>
        <td class="tdr">5569</td>
        <td class="tdr">5390</td>
        <td class="tdr">5233</td>
        <td class="tdr">5189</td>
        <td class="tdr">5004</td>
        <td class="center">4694<br />to<br />4629</td>
        <td>band.</td>
      </tr>
      <tr>
        <td>Hydrogen</td>
        <td class="tdr">5555</td>
        <td class="tdr">5422</td>
        <td class="center">...</td>
        <td class="tdr">5189</td>
        <td class="tdr">5008</td>
        <td class="center">4632</td>
        <td></td>
      </tr>
    </table>

    <p>I remarked that a line (5596) described by Dr. Vogel as “very bright”
    in his H spectrum does not appear in my tube, though in most other respects
    our H spectra agree.</p>

    <div class="sidenote">Effect of distance
    on the
    spectrum.</div>

    <p>I thought this tube afforded a good opportunity for testing the effect of
    distance upon the spectrum. The slit was made rather fine. At 6 inches
    distance from it the line α in the blue-green (F solar) was very bright. The
    lines marked β, γ, δ, ε, and ζ also survived, but were faint. At 12 inches
    from the slit α and γ were alone seen, and at 24 inches α stood by itself
    upon a dark ground. I also noticed that the red and yellow parts of the
    spectrum first lost their light on the tube being withdrawn from the slit;
    and this appeared to account for β disappearing while γ survived. For
    drawing of the hydrogen-tube spectrum see Plate XIV. spectrum 1.</p>

    <p>The question of effect of distance upon the spectroscopic appearance of a
    glowing light, as tested for this and other tubes, seems an important one. It
    may possibly account for the generally faint aspect of the lines in the more
    refrangible part of the Auroral spectrum.</p>

    <figure class="plate" id="plate14">
      <!-- <img src="assets/aurorae/images/plate14.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-11-2"><i>Carbon- and Oxygen-tubes.</i></h4>

    <div class="sidenote">Carbon- and
    oxygen-tubes. Tubes described. Carbon-tubes
    lighted
    up. Spectra of
    the carbon-tubes
    described.</div>

    <p>The following tube-observations were taken together, because my friend
    Mr. Henry R. Procter (to whom I am indebted for many profitable hints and
    suggestions in Auroral work) was disposed to regard the spectra found in the
    carbon-tubes and in those marked “O” as identical, suggesting that pure O,
    with the ordinary non-intensified discharge, gives only a continuous spectrum;
    and that the O tubes are in fact generally lighted up by a carbon-spectrum,
    as the result of impurity from accidental causes. The tubes examined for the
    purpose of comparison were as follows:—A coal-gas tube, a tube marked
    “C.A.,” three O tubes (two of, I believe, London make, and the third from
    Geissler), and an OH₂ tube, also from Geissler. The carbon-tubes were both
    brilliantly and steadily lighted by the current. The C.A. tube glowed with
    a peculiar silvery-grey green light in the capillary part, and with a grey glow,
    considerably stratified, in the bulbs. The coal-gas tube illumination was
    whiter and still more brilliant than the C.A., and with even finer stratification
    in the bulbs. The spectra of both tubes were conspicuous for the same
    three well-known principal bright lines or bands in the yellow, green, and
    blue (with one fainter in the violet), all shading off towards the violet, and
    in both cases with fainter intervening bands or lines. These last bands or
    lines only partially coincided when the two tubes were compared.</p>

    <p>The spectra in both cases were rich and glowing, with a certain amount
    of continuous spectrum between the lines; and the three principal bands
    or lines showed well and distinctly their respective place-colours.</p>

    <p></p>

    <div class="sidenote">Tubes tested
    for distance.</div>

    <p><i>Tubes tested for distance.</i>—In the case of the C.A. tube, at 18 inches from
    the slit the continuous spectrum and fainter lines disappeared, while the
    four principal lines still shone out, that in the green being the strongest. At
    24 inches the same lines were still visible, though somewhat faintly.</p>

    <p>In the case of the coal-gas tube, at 24 inches the whole spectrum was
    quite brilliant, the four principal lines being very bright and even preserving
    their distinctive colours. The H line, near the line or band in the
    blue, was also plainly seen.</p>

    <div class="sidenote">O tubes
    lighted up.</div>

    <p>The O tubes, when treated by the same current as the carbon-tubes, were
    found to be all three identical in general features. The discharge lighted up
    each of the tubes feebly and somewhat intermittently. Grey in the bulbs
    and a faint but decidedly pinkish white in the capillary part were the distinguishing
    light colours; while nothing could be more marked than the difference
    in brilliancy between these and the preceding carbon-tubes.</p>

    <div class="sidenote">OH₂ tubes
    lighted up. O tubes
    spectra described.</div>

    <p>The OH₂ tube presented very much the same character; but the discharge
    occasionally varied from a pinkish white to a yellow colour, somewhat like
    that which artists call brown-pink, and reminding one of the “golden rays”
    in certain Auroræ. These O spectra presented, in common with the carbon-tubes,
    three principal bright lines or bands in the yellow, green, and blue,
    with a fainter one in the violet, all shading off towards the violet. The bands,
    however, showed but very little trace of local colour, and the whole spectrum
    had a faint and washed-out look, very different from the carbon-spectra. (I
    certainly, by a little management, subsequently succeeded in getting the
    same look to the C.A. spectrum; but it was only by removing the tube to
    some distance from the slit, and thus depriving the spectrum of very much of
    its brightness.) The hydrogen line (solar F) was bright, more so than any of
    the O lines.</p>

    <p>The intensity of the three principal lines seemed to me to run in the
    following order:—</p>

    <table summary="Intensity">
      <tr>
        <th></th>
        <th>Yellow.</th>
        <th>Green.</th>
        <th>Blue.</th>
      </tr>
      <tr>
        <td>Coal-gas</td>
        <td class="center">β</td>
        <td class="center">α</td>
        <td class="center">γ</td>
      </tr>
      <tr>
        <td>Oxygen</td>
        <td class="center">γ</td>
        <td class="center">α</td>
        <td class="center">β</td>
      </tr>
    </table>

    <p class="noindent">Between the lines γ and α in the Geissler O tube I found a rather bright
    line, which I shall have occasion to refer to hereafter.</p>

    <p>At 12 inches distance from the slit the O spectrum lost nearly all its light;
    the H line, and the three lines γ, α, and β, alone faintly remaining, α being
    decidedly the brightest. At 24 inches no spectrum at all was to be seen.</p>

    <p></p>

    <div class="sidenote">Comparison
    of spectra
    of coal-gas
    and O tubes.</div>

    <p>I carefully compared together the three principal lines of the two spectra
    of coal-gas and O by means of:—</p>

    <p>1st, the photographed micrometer before described;</p>

    <p>2nd, a comparison-prism on the slit plate;</p>

    <p>3rd, a piece of very fine brass foil cut as a pointer and fixed in the focus
    of a positive eyepiece.</p>

    <p>The lines or bands in both tubes were found to be slightly nebulous
    towards the less-refrangible end (where they were measured), and the O tube
    was not bright under a moderately high power (positive eyepiece). Subject
    to these remarks, the three principal lines in both tubes were found to correspond
    in position within the limits of my instrument. The spectra did not,
    however, I am bound to say, <i>look</i> alike.</p>

    <div class="sidenote">Dr. Vogel’s
    O spectrum
    reduced and
    compared.</div>

    <p>Puzzled by these observations, it then occurred to me to reduce Dr. Vogel’s
    spectrum of O, given in his memoir, to the same scale with my own. This
    I did independently, and I then compared the result with my own spectrum
    as mapped out. From the comparison, I judge that if my O tubes, one and
    all, showed a carbon-spectrum, the learned Doctor’s tube must have been
    subject to a similar infirmity, as the tubes all agreed in main features.</p>

    <p>There is, however, one point to which I desire to draw attention, which is
    this, that common to both the Doctor’s and my own Geissler spectrum
    I found the before-mentioned rather bright line between γ and α. This
    line I found no equivalent for in either of the carbon-tubes. For spectra
    of coal-gas and oxygen-tubes, see Plate XIV. spectra 2, 3, &amp; 4.</p>

    <div class="sidenote">Tube- and
    flame-spectra
    of carbon
    do not
    correspond.</div>

    <p>In comparing the spectra, it should be remembered that the tube- and
    flame-spectra of carbon do not correspond. Compare, for instance, the
    spectrum of coal-gas or CO₂ in tube, and the well-known lines or bands in
    the blue base of a candle-flame. The sharper edge of the yellow line or
    band of the carbon-tubes will be found about midway between the two bright
    yellow candle-lines or bands. The first of the very beautiful group of lines
    or bands in the green in the candle-flame falls considerably behind the
    sharper edge of the green line or band in the tube, while the third bright
    band in the tube, alone of the three, corresponds with a very faint band in
    the candle-flame. A line or band in the violet in the tube-spectrum finds no
    equivalent in the candle-spectrum. For comparison of the carbon-tube and
    flame spectra (the principal lines of the tube being alone shown), see Plate
    XVI. spectra 6 &amp; 7.</p>

    <div class="sidenote">Prof. Piazzi
    Smyth’s
    measurements
    of
    the components
    of
    the citron-band
    in a
    coal-gas
    flame.</div>

    <p><i>Note.</i>—Prof. Piazzi Smyth has been good enough, at my instance, to
    measure the components of the citron band of the carbo-hydrogen spectrum
    (near Ångström’s Aurora-line), as seen in a coal-gas blowpipe-flame urged
    with common air.</p>

    <p>The spectroscope used had prisms giving 22° of dispersion between A and H,
    and the observing telescope magnified 10 times. The following is a table
    of the results communicated to me by the Professor:—</p>

    <table summary="Results of the experiment">
      <tr>
        <th></th>
        <th></th>
        <th>Intensity.</th>
        <th>Reading of<br />Micrometer.</th>
      </tr>
      <tr>
        <td colspan="2">Reference line, lithium β</td>
        <td class="tdr">4</td>
        <td class="tdr">16·55</td>
      </tr>
      <tr>
        <td colspan="2"><span class="ditto3">”</span> sodium, α1</td>
        <td class="tdr">10</td>
        <td class="tdr">18·45</td>
      </tr>
      <tr>
        <td colspan="2"><span class="ditto3">”</span> <span class="ditto2">”</span> α2</td>
        <td class="tdr">10</td>
        <td class="tdr">18·51</td>
      </tr>
      <tr>
        <td class="center" colspan="4">Citron band. Carbo-hydrogen.</td>
      </tr>
      <tr>
        <td class="tdr">Line 1,</td>
        <td>exquisitely clear</td>
        <td class="tdr">6</td>
        <td class="tdr">21·28</td>
      </tr>
      <tr>
        <td class="tdr">2,</td>
        <td><span class="ditto2">”</span></td>
        <td class="tdr">5</td>
        <td class="tdr">21·88</td>
      </tr>
      <tr>
        <td class="tdr">3,</td>
        <td><span class="ditto2">”</span></td>
        <td class="tdr">3</td>
        <td class="tdr">22·44</td>
      </tr>
      <tr>
        <td class="tdr">4,</td>
        <td>faint but clear</td>
        <td class="tdr">2</td>
        <td class="tdr">22·95</td>
      </tr>
      <tr>
        <td class="tdr">5,</td>
        <td>faint</td>
        <td class="tdr">1</td>
        <td class="tdr">23·38</td>
      </tr>
      <tr>
        <td class="tdr">6,</td>
        <td>faint and hazy</td>
        <td class="tdr">1</td>
        <td class="tdr">23·70</td>
      </tr>
      <tr>
        <td class="tdr">7,</td>
        <td>doubtful</td>
        <td class="tdr">?</td>
        <td class="tdr">23·92</td>
      </tr>
      <tr>
        <td colspan="2">Reference line, thallium α</td>
        <td class="tdr">10</td>
        <td class="tdr">25·08</td>
      </tr>
    </table>

    <div class="sidenote">Comparison
    of Dr.
    Vogel’s O
    lines and
    Dr. Watts’s
    carbon-lines.</div>

    <p>From Dr. Watts’s ‘Index of Spectra’ I have extracted the three principal
    carbon-tube bands or lines; and they compare with Dr. Vogel’s oxygen-tube
    as under:—</p>

    <table summary="Comparison of Vogel's and Watts' lines">
      <tr>
        <th></th>
        <th>Yellow.</th>
        <th>Green.</th>
        <th>Blue.</th>
      </tr>
      <tr>
        <td>Dr. Vogel’s oxygen-lines</td>
        <td class="tdr">5603</td>
        <td class="tdr">5189</td>
        <td class="tdr">4829</td>
      </tr>
      <tr>
        <td>Dr. Watts’s carbon-tube bands or lines</td>
        <td class="tdr">5602</td>
        <td class="tdr">5195</td>
        <td class="tdr">4834</td>
      </tr>
    </table>

    <p>Now these wave-length differences are so small that they raise a presumption
    of the possibility of the spectra being identical. On the other hand,
    assuming the spectra are not identical, the comparison tells the other way,
    viz. that the differences are so minute as to escape detection in instruments
    of moderate dispersion. With my own instrument I found the O spectrum
    too faint to increase the dispersive power with advantage. Considering the
    extremely different character of the two discharges, the identity of all the
    O tubes, and the presence of the line found between γ and α in the
    O spectrum, I think the two spectra are independent, though I admit there
    is room for doubt.</p>

    <div class="sidenote">O and CO₂
    spectra photographed.</div>

    <p><i>Note.</i>—Since this examination I have photographed both spectra side by
    side (see ‘Photographed Spectra,’ Plate XXXI., text, pp. 69, 70). The
    pictures include, of course, only the blue and violet parts of the spectrum;
    but they are widely different in aspect, and show that, photographically at
    least, in this part of the spectrum there is a complete want of identity.
    Subsequent investigations, however, by Schuster and others (detailed later in
    this Chapter), go to establish that the principal lines shown in mine and
    Dr. Vogel’s tubes were due to (probably hydrocarbon) impurity. The exception
    is the single line common to mine and Dr. Vogel’s tubes, but absent
    from the coal-gas spectrum. This line proves to be oxygen. Compare
    oxygen-tube spectra (Plate XIV. spectra 3 and 4) with Schuster’s oxygen-tube
    spectrum (Plate XVIII. fig. 15). The line in question is found identical in
    the three tubes.</p>

    <p>The tube OH₂ was found to give the principal lines of the O and H spectra
    combined on a faint continuous spectrum.</p>

    <h4 id="chap-11-3"><i>Geissler Mercury-tube</i> (Plate X. fig. 7) <i>and Barometer
    Mercurial vacuum</i>.</h4>

    <div class="sidenote">Mercury-
    and barometer-tubes
    examined. Mercury-tube
    described. Barometer-tube.</div>

    <p>I next examined two vacuum-tubes of an entirely different character. The
    one was a tube from Geissler of stout glass, some fifteen inches long,
    without electrodes, and an inch across. Within this tube was a second of
    uranium glass, with bulbs blown in it. In contact with both tubes a quantity
    of fluid mercury ran loose (Plate X. fig. 7). Upon shaking this tube with
    the hand brilliant flashes of blue-white light, like summer lightning, flashed
    out. These were discernible (though faintly) even in daylight. The fine
    terminal wires of the coil being wrapped round each end of this tube, when
    the current passed, a bright and white induced discharge, with a considerable
    amount of stratification, was seen in the tube. The other tube was that of a
    mercurial siphon-barometer. This being placed in a stand, one terminal
    wire was placed in the mercury in the short leg of the siphon, while the other
    terminal was made into a little coil and placed on the upper closed extremity
    of the barometer-tube. On passing the current, the entire short space above
    the mercury was filled with a grey-white light, not stratified, but showing a
    conspicuous bright ring just above the level of the mercury.</p>

    <div class="sidenote">Spectrum
    of both
    these tubes
    described.</div>

    <p>Both these tubes, when examined with the spectroscope, showed four
    bright rather uniform bands (the central one being the brightest), which I
    assigned to the carbon-spectra (see Plate XIV. spectra 5 and 6).</p>

    <p>The Geissler tube was probably filled designedly with coal-gas. In the
    case of the barometer-tube the spectrum must be assumed to be the result of
    some carbon impurity.</p>

    <p></p>

    <p>No lines of mercury could be detected in either case.</p>

    <p>An effort was made to examine the light of the Geissler mercury-tube as
    excited by motion only, but the spectrum could not be kept in the field;
    the four lines were, however, seen to flash out as the light passed before
    the slit.</p>

    <figure class="plate" id="plate15">
      <!-- <img src="assets/aurorae/images/plate15.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <h4 id="chap-11-4"><i>Air-tubes.</i></h4>

    <div class="sidenote">Air-tube
    illuminated.</div>

    <p>The first tube I examined was an ordinary Geissler tube charged with
    rarefied air. The bulbs, on passing the discharge, were filled with the well-known
    rose-tinged light like to the Aurora-streams. This in the capillary
    part was condensed into a brighter and whiter thread, while the platinum
    wire of the negative pole was surrounded by its characteristic mauve or
    violet glow.</p>

    <div class="sidenote">Spectrum
    described.</div>

    <p>The spectrum, even with a weak current, was quite bright, and consisted
    mainly of the nitrogen-lines and bands, with the lines Hα, Hβ, and Hγ, and
    some of the intermediate lines of the H tube.</p>

    <p>The double line α was undoubtedly the brightest in the spectrum when
    taken in the capillary part of the tube. After this followed β, and then γ(H),
    δ, and ε. I was, however, uncertain as to the relative brightness of the last
    three, and marked their intensities with hesitation. I tested them several
    times independently with differing results, and suspected them of variability
    with the current.</p>

    <p>The rest of the lines were very much of the same intensity. (For drawing
    of spectrum of air-tube in capillary part see Plate XV. spectrum 1.)</p>

    <h4 id="chap-11-5"><i>Violet [negative] Pole, same tube.</i></h4>

    <div class="sidenote">Violet
    (negative)
    pole: spectrum
    described.</div>

    <p>I next turned my attention to the violet or negative-pole glow; and here a
    remarkable change took place in the spectrum, not only in the position of
    the principal bands or lines, but in their relative intensity (see Plate XV.
    spectrum 2).</p>

    <p>The double line α in the capillary part was replaced in the violet glow by
    a shaded band of second intensity β, the sharp edge of which was extended
    towards the red, and formed (except for some faint indications) the limit of
    the spectrum in that direction. The somewhat faint line next α in the
    capillary tube had its faint representative in the violet pole; but the next two
    lines (capillary) were represented by the bright band γ in the violet pole
    lying in a position between them. Next γ in the violet pole came three
    faint lines, representing β, γ, and δ in the capillary spectrum; and then the
    bright band α, which was the brightest of the violet-pole group, and represented
    a medium-intensity band in the capillary spectrum. After this was a
    faint band near α, representing two rather bright ones in the capillary
    spectrum, this last being succeeded by other bands in the violet. α, β, and γ
    in the violet pole were examined carefully for relative brightness, and were,
    I believe, correctly marked.</p>

    <h4 id="chap-11-6"><i>Red [positive] Pole.</i></h4>

    <div class="sidenote">Red (positive)
    pole:
    spectrum
    described.</div>

    <p>The red [positive] pole was next examined, but presented no peculiar
    features. It appeared as a fainter representation of the capillary air-spectrum,
    with some few lines or bands absent, and (as will be seen after) was also a fair
    representation of a diffused air-spectrum (see Plate XV. spectrum 3).</p>

    <p>Examined for comparative intensity, at 24 inches from the slit, the whole
    capillary air-spectrum showed faintly. The marked lines in the centre of the
    spectrum generally retained their prominence; but after α I judged ε next in
    brightness. On examining the violet pole at 12 inches from the slit, the
    whole spectrum was faint and the bands α and β were alone distinctly seen.</p>

    <h4 id="chap-11-7"><i>Aurora (air)-tube.</i> (Plate XV. spectrum 4.)</h4>

    <div class="sidenote">Aurora-tube:
    discharge
    described. Spectrum
    described.</div>

    <p>Next to the Geissler air-tube I examined an “aurora”-tube, about
    15 inches long and 1¼ inch across, with platinum terminals, and of the same
    diameter throughout (Plate X. fig. 8). The discharge was of a rosy-red
    colour, and the long flickering stream from pole to pole certainly much
    reminded one optically of an auroral streamer. Spectroscopically examined,
    the discharge presented a faint banded air-spectrum similar to that of the
    positive pole (see Plate XV. spectrum 4); but the relative intensity of the
    lines was somewhat altered, while a very bright line in the green (seen also in
    the tube next described) was characteristic of the spectrum, and in this respect
    distinguished it from the ordinary air-spectrum.</p>

    <h4 id="chap-11-8"><i>Phosphorescent tube.</i></h4>

    <div class="sidenote">Phosphorescent
    tube
    described. Discharge
    described. Spectrum
    described.</div>

    <p>Following this last tube I examined one purchased as “phosphorescent.”
    It was rather short (6½ inches), of equal calibre, and about the size of the
    bulb of a Geissler tube. It was filled with a white powder (probably one of
    the Becquerel compounds). On passing the current between the electrodes,
    a bright rose-coloured stream appeared; and wherever this was in contact with
    the powder, the tube glowed with a brilliant green light. On stopping the
    current, the tube still continued to shine, but with a fainter green glow, which
    gave only a continuous spectrum. When examined in full glow, the tube-spectrum
    was also in the main continuous and of a green tinge; but upon it
    were bright lines in the blue and violet portions of the spectrum, while in
    the red, yellow, and green a faint but distinct air-spectrum was seen; and
    with this was also found the same bright line in the green which distinguished
    the “aurora”-tube. [Five out of six of the lines in the blue and
    violet will be also found in Schuster’s oxygen-tube, violet pole (Plate XVIII.
    fig. 15). The air-spectrum probably arose from impurity.]</p>

    <figure class="plate" id="plate16">
      <!-- <img src="assets/aurorae/images/plate16.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <p></p>

    <h4 id="chap-11-9"><i>Spark in Air.</i></h4>

    <div class="sidenote">Spark in air:
    spectrum
    described.</div>

    <p>I next took a ½-inch spark in air between platinum terminals (see Plate XV.
    spectrum 6). The principal lines in this spectrum were the line α (by far the
    brightest), corresponding to γ in the violet pole; next was β, a line in the
    yellow, not appearing in the tube-spectrum, and then other lines of less
    intensity. In the “aurora” and “phosphorescent” tubes was found, as before
    mentioned, a line in the green prominent for its brightness, and, indeed, in
    the “aurora”-tube the only one which survived when it was moved away
    from the slit. This line also appeared in the spark-spectrum, but there only
    of an average brightness. I examined it carefully for position in the respective
    tubes; and on comparing them by means of a pointer in the eyepiece, found
    it coincident with the ridge or centre of the wedge-like bright-green broad
    band which is so conspicuous in the air-tube spectrum.</p>

    <p>I think this edge-like centre has actually a line coincident with the line I
    refer to; but if so, its intensity little exceeds that of the band itself.</p>

    <h4 id="chap-11-10"><i>Spark over Water.</i></h4>

    <div class="sidenote">Spark over
    water:
    spectrum
    described.</div>

    <p>To complete the set of air-experiments, I examined the same spark taken
    from the surface of a small meniscus of water, placed in a glass cup upon the
    lower platinum wire. In this case the air-spectrum was plainly, but not
    brightly, seen at the violet end of the spectrum—the red, yellow, green, and
    blue being filled with a continuous spectrum, through which some of the
    air-lines faintly showed (see Plate XV. spectrum 7).</p>

    <h4 id="chap-11-11"><i>Phosphoretted-Hydrogen Flame.</i></h4>

    <div class="sidenote">Phosphoretted-hydrogen
    flame.</div>

    <p>This was obtained from a hydrogen-bottle fitted with glass tubing, two or
    three minute pieces of phosphorus being placed with the zinc. The flame was
    of a bright yellow colour, with a cone of vivid green light in its centre.</p>

    <div class="sidenote">Spectrum
    described.</div>

    <p>The spectrum was found to consist mainly of three bright bands in the
    yellow, green, and green-blue respectively (see Plate XVI. spectrum 3).</p>

    <div class="sidenote">Mons. Lecoq
    de Boisbaudran’s
    remarks
    on the
    spectrum
    increasing
    in brilliancy
    when the
    flame is
    cooled.</div>

    <p>The central band was very striking in its emerald-green colour, while all
    the bands were remarkable as being very broad in proportion to the slit
    (which, however, was not fine). The yellow band had a rich glow of colour.
    My spectrum was mapped out at ordinary temperature, and I found the bands
    sufficiently bright; but Mons. Lecoq de Boisbaudran, in his ‘Spectres
    Lumineux’ (texte, p. 188), has described how the brilliancy of these bands is
    increased when the flame is artificially cooled (<i>refroidie</i>).</p>

    <p>The idea of cooling the flame was due to M. Salet, who effected it either
    by a jet of water or by an air-blast.</p>

    <p>The less refrangible bands seem the most susceptible to increase of brilliancy.</p>

    <p>Mons. Boisbaudran also makes the important remark that the relative
    intensities of the bands are in such case altered, adding:—“La plus importante
    de ces modifications consiste en un renforcement très-considérable de la bande
    rouge δ 97·03 (W.L. 5994) qui devient vive de presque invisible qu’elle était
    en l’absence du refroidissement artificiel de la flamme.”</p>

    <p>Full details of the changes are given by M. de Boisbaudran.</p>

    <p>The bearing of these observations as connected with the variable character
    of the red line in the Aurora-spectrum seems to me in the highest degree
    noteworthy.</p>

    <h4 id="chap-11-12"><i>Iron-Spectrum.</i></h4>

    <span class="sidenote">Iron-spectrum.</span>

    <p>A comparison of this spectrum suggested itself, partly from the suspected
    relations between the Aurora and solar corona, and partly from a consideration
    of the views expressed by M. Gronemann and others in favour of the Aurora
    having its origin in the fall of an incandescent meteoric powder.</p>

    <div class="sidenote">How
    obtained. Spectrum
    described. Mons. Lecoq
    de Boisbaudran’s
    spectra also
    given.</div>

    <p>The spectrum was obtained from a spark taken over a solution of perchloride
    of iron in a small glass cup, and was remarkable for its brightness in and
    about the green region. The lines varied considerably in intensity, and with
    a fine slit the principal ones were sharp, distinct, and clear. A group of three
    lines (α) stood out boldly in the green as the most marked, and next to these
    a group of three others more towards the violet end of the spectrum (see
    Plate XVI. spectrum 4). By the side of my phosphoretted-hydrogen and
    iron spectra I have placed the principal lines of Mons. Lecoq de Boisbaudran’s
    same spectra (reduced to my scale), and with figures of wave-lengths for comparison
    with the Aurora-spectrum (see Plate XVI. spectra 1 and 2).</p>

    <div class="sidenote">Comparison
    of iron- and
    Aurora-spectrum.</div>

    <p>A difficulty in comparing the iron-spectrum with that of the Aurora arises
    from the large number of fine lines found in the former spectrum. In a photograph
    (taken with the same prism as before described) of a small piece of
    meteoric iron fused in an electric arc by the aid of 40 Grove cells, about
    154 lines are easily counted in the blue and violet parts of the spectrum.
    Double this number at least would be seen with a spectroscope of moderate
    dispersion in the region comprising the entire set of auroral lines.</p>

    <p></p>

    <h4 id="chap-11-13"><i>Spectrum of Mercury.</i></h4>

    <div class="sidenote">Mercury-spectrum. How
    obtained.</div>

    <p>This spectrum is given as useful for comparison with the bright and
    principal Aurora-line. It is easy to obtain with a small coil, the metal
    being used as one electrode. The yellow lines are distinct and steady; but
    the green, which is very bright, is apt to flicker as the spark moves on the
    surface of the metal (see Plate XVI. spectrum 5).</p>

    <h4 id="chap-11-14"><i>The following Table was compiled for the purpose of comparing the
    foregoing results with the Aurora-spectrum.</i></h4>

    <span class="sidenote">Table of coincidences.</span>

    <p><span class="smcap">Table</span> showing comparative position of Aurora-lines with the principal lines
    in the examined spectra. C. means coincident within the limits of my
    instrument and scale, N. near, and VN. very near.</p>

    <table summary="Table of coincidences" class="borders">
      <tr>
        <th class="nw">Aurora-lines</th>
        <th class="nw">6297 β.</th>
        <th class="nw">5569 α.</th>
        <th class="nw">5390 ζ.</th>
        <th class="nw">5233 δ.</th>
        <th class="nw">5180 δ.</th>
        <th class="nw">5004 γ.</th>
        <th>4694 to<br />4629 ε.</th>
        <th class="nw">4350? ε.</th>
      </tr>
      <tr>
        <td class="bl bb">Hydrogen-tube</td>
        <td rowspan="11" class="bb">No results in the examined spectra; but see Plate XIII. fig. 2.</td>
        <td class="bb">N.</td>
        <td class="bb"></td>
        <td class="bb">N.</td>
        <td class="bb">C., same W.L.</td>
        <td class="bb"></td>
        <td class="bb">Band includes 2 lines.</td>
        <td rowspan="11" class="bb">Too uncertain in position for comparison (see Plate XIII. fig. 1).</td>
      </tr>
      <tr>
        <td class="bl bb">Coal-gas tube</td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb">N.</td>
        <td class="bb">VN.</td>
        <td class="bb"></td>
        <td class="bb">Band includes 1 line.</td>
      </tr>
      <tr>
        <td class="bl bb">Oxygen-tube</td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb">VN.</td>
        <td class="bb"></td>
        <td class="bb"></td>
      </tr>
      <tr>
        <td class="bl bb">Air, capillary</td>
        <td class="bb">Band includes</td>
        <td class="bb">Band includes</td>
        <td class="bb"></td>
        <td class="bb">N.</td>
        <td class="bb"></td>
        <td class="bb">Band includes 2 lines.</td>
      </tr>
      <tr>
        <td class="bl bb">Air, violet-pole</td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb">Band includes</td>
        <td class="bb">C.</td>
        <td class="bb">Band includes 1 line.</td>
      </tr>
      <tr>
        <td class="bl bb">Air, red-pole</td>
        <td class="bb" colspan="6">See Air, capillary.</td>
      </tr>
      <tr>
        <td class="bl bb">Aurora-tube&nbsp;and<br />phosphorescent&nbsp;tube</td>
        <td class="bb" colspan="6">See Air, capillary; and note bright line.</td>
      </tr>
      <tr>
        <td class="bl bb">Air, spark</td>
        <td class="bb">N.</td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb">N.</td>
        <td class="bb">C.</td>
        <td class="bb">Band includes 2 lines.</td>
      </tr>
      <tr>
        <td class="bl bb">Air, spark over water</td>
        <td class="bb" colspan="5">Continuous spectrum and faint air-lines.</td>
        <td class="bb">Band includes 2 lines.</td>
      </tr>
      <tr>
        <td class="bl bb">Phosphoretted hydrogen</td>
        <td class="bb">N.</td>
        <td class="bb">Faint band.</td>
        <td class="bb">Band includes</td>
        <td class="bb"></td>
        <td class="bb"></td>
        <td class="bb"></td>
      </tr>
      <tr>
        <td class="bl bb">Iron</td>
        <td class="bb">VN.</td>
        <td class="bb">N.</td>
        <td class="bb">VN.</td>
        <td class="bb">VN.</td>
        <td class="bb"></td>
        <td class="bb"></td>
      </tr>
    </table>

    <p></p>

    <p>Tested by coincidence, or close proximity of lines to those of the Aurora,
    we arrange the spectra in the following order:—(1) iron, (2) air-spark,
    (3) hydrogen, (4) air-tube, (5) phosphoretted hydrogen, (6) carbon and oxygen.</p>

    <p>The air-tube spectrum might perhaps stand higher in the scale but for
    its broad bands, which make comparison doubtful. Lines of oxygen possibly
    escape detection in the Aurora from the faint character of its spectrum.</p>

    <p>The phosphorus and iron spectra are especially interesting in connexion
    with Professor Nordenskiöld’s “metallic and magnetic cosmic dust in the
    Polar regions” (see Phil. Mag. ser. 4, vol. xlviii. p. 546).</p>

    <div class="sidenote">Additional
    Table of
    compared
    spectra.</div>

    <p>As an addendum to the foregoing, on Plate IX. fig. 1 will be found a
    Table I have prepared, in which a type Aurora and also Vogel’s and
    Barker’s Auroræ are compared with eight other spectra, viz.:—</p>

    <table summary="The spectra">
      <tr>
        <td>S.</td>
        <td>Solar spectrum.</td>
      </tr>
      <tr>
        <td>N.</td>
        <td>Nitrogen (air): Watts.</td>
      </tr>
      <tr>
        <td>O.</td>
        <td>Oxygen (air): Watts.</td>
      </tr>
      <tr>
        <td>C.H.</td>
        <td>Carburetted-hydrogen vacuum-tube: Watts.</td>
      </tr>
      <tr>
        <td>C.I.</td>
        <td>Carburetted-hydrogen flame: Watts.</td>
      </tr>
      <tr>
        <td>C.C.</td>
        <td>Blue base of candle-flame: Capron.</td>
      </tr>
      <tr>
        <td>O.P.</td>
        <td>Oxygen vacuum-tube: Procter.</td>
      </tr>
      <tr>
        <td>I.</td>
        <td>Iron: Watts.</td>
      </tr>
    </table>

    <p>The divisions and vertical lines will guide the eye in making comparison of
    the spectra.</p>

  </section>

  <!-- Chapitre 12_________________________________________________________-->
  <section class="chapter" id="chapter-12">
    <h3 class="titlechapter" id="chap-12">Notes on Professor Ångström’s Theory of the Auroraspectrum</h3>
    <p class="shorter">Notes on Professor Ångström’s Theory</p>

    <p class="smaller">[The substance of these appeared in the ‘Philosophical Magazine’ for April 1875, in conjunction
    with the “Comparison of the Tube and other Spectra” (Chapter XI.), but they are now,
    for the sake of convenience, made a separate article.]</p>

    <div class="sidenote">Professor
    Ångström’s
    propositions.</div>

    <p>In a contribution by the late Professor Ångström to a solution of the problem
    of the Aurora-spectrum (an abstract of which appeared in ‘Nature’ of
    July 16, 1874), the Professor is stated, amongst other things, to have laid
    down certain propositions in substance as follows:—</p>

    <div class="sidenote">That the
    Aurora has
    two spectra.</div>

    <p>1st. That the Aurora has two different spectra—the one comprising the
    one bright line in the yellow-green only, and the other the remaining fainter
    lines.</p>

    <div class="sidenote">That bright
    line does not
    coincide
    with HC₂.</div>

    <p>2ndly. That the bright line falls within a group of hydrocarbon lines, but
    does not actually coincide with any prominent line of such group, and that
    Dr. Vogel’s finding this line to coincide with a not well-marked band in the
    air-spectrum must be regarded as a case of accidental coincidence.</p>

    <div class="sidenote">That moisture
    is <i>nil</i>
    in Aurora
    region.</div>

    <p>3rdly. That moisture in the region of the Aurora must be regarded as
    <i>nil</i>, and that oxygen and hydrogen must alone there act as conductors of
    electricity.</p>

    <div class="sidenote">Ångström’s
    flask-experiment
    described.</div>

    <p>Professor Ångström then details the examination of an exhausted dry air-flask
    filled with a discharge analogous to the glow of the negative pole of a
    vacuum air-tube.</p>

    <div class="sidenote">Flask-spectrum
    compared
    with
    Aurora-spectrum.</div>

    <p>The experiment is described as follows:—“Into a flask, the bottom of
    which is covered with a layer of phosphoric anhydride, the platinum wires
    are introduced, and the air is pumped out to a tension of only a few millimetres.
    If the inductive current of a Ruhmkorff coil be sent through the
    flask, the whole flask will be filled, as it were, with a violet light, which otherwise
    only proceeds from the negative pole, and from both electrodes a spectrum
    is obtained composed chiefly of shaded violet bands.” The comparison of
    the spectrum of this violet glow with that of the Aurora gives, according to
    Ångström, the following results:—</p>

    <table summary="Comparison of violet and aurora">
      <tr>
        <td>Aurora-lines, wave-lengths</td>
        <td class="tdr">4286</td>
        <td class="tdr">4703</td>
        <td class="tdr">5226</td>
      </tr>
      <tr>
        <td>Violet light, wave-lengths</td>
        <td class="tdr">4272</td>
        <td class="tdr">4707</td>
        <td class="tdr">5227</td>
      </tr>
    </table>

    <p></p>

    <p>Two weak light bands, found by Dr. Vogel at 4663 and 4629, are also
    compared with other lines in the violet light 4654 and 4601; and the Professor
    then concludes that it may be in general assumed that the feeble
    bands of the Aurora-spectrum belong to the spectrum of the negative pole,
    possibly changed more or less by additions from the banded or the line air-spectrum.</p>

    <div class="sidenote">Bright line
    is due to
    fluorescence
    or phosphorescence.</div>

    <p>4thly. That the only probable explanation of the bright line is, that it
    owes its origin to fluorescence or phosphorescence. The Professor remarks on
    this point that “an electric discharge may easily be imagined which, though
    in itself of feeble light, may be rich in ultra-violet light, and therefore in a
    condition to cause a sufficiently strong fluorescence.” He notes also that
    oxygen and some of its compounds are fluorescent.</p>

    <div class="sidenote">No need of
    Dr. Vogel’s
    theory of
    variability.</div>

    <p>5thly. That there is no need, in order to account for the spectrum of the
    Aurora, to have recourse to the “very great variability of gas-spectra according
    to the varying circumstances of pressure and temperature” (Dr. Vogel’s
    theory). Professor Ångström does not admit such variability, and does not
    admit that the way a gas may be brought to glow or burn can alter the nature
    of the spectrum.</p>

    <div class="sidenote">Professor
    Ångström’s
    conclusions
    tested.</div>

    <p>In order to test some of the Professor’s conclusions in an experimental way,
    I examined some tube and other spectra not only for line-positions, but also
    for general resemblance to an Aurora-spectrum.</p>

    <p>These experiments are detailed in the last Chapter, and the results are
    comprised in Plates XIV., XV., and XVI., in which the spectra obtained are
    represented in black for white.</p>

    <div class="sidenote">Result of
    examination
    of the Professor’s
    propositions.</div>

    <p>The result of the examination of Professor Ångström’s principal propositions
    seems to be this:—</p>

    <p>1st. Two Auroral spectra. I agree in this, but question whether the fainter
    lines may not possibly comprise more than one spectrum.</p>

    <p>2nd. I agree also that the bright yellow-green line falls, as Professor Ångström
    describes, just behind the second line in the hydrocarbon yellow group
    (see Plate V. fig. 7). And I find, in common with the Professor, no well-marked
    or prominent line in the air-spectrum with which it accords.</p>

    <p>3rd. This may be conveniently divided into two parts, viz.:—</p>

    <p>A. The proposition that “moisture in the region of the Aurora must be
    regarded as <i>nil</i>.”</p>

    <div class="sidenote">Moisture
    probably not
    <i>nil</i> in the
    Aurora region. Reasons for
    this given. Aurora in
    vapour or
    mist. Frequently
    near to
    earth’s surface.</div>

    <p>Here I see reason to differ, since (to quote a letter of Mr. Procter’s) “the
    vapour-density of OH₂ is only 9 against 14 for N and 16 for O;” and again,
    “electrical or heat-repulsion may carry water-dust up to enormous heights.”
    There are, too, I think, circumstances connected with the Aurora itself which
    make the assumption of moisture being <i>nil</i> in the Auroral regions untenable.
    The first of these is the fact that the white arc, streamers, and floating
    patches of light, found in some Auroræ, have frequently the peculiarly dense
    and solid look of vapour-clouds—a circumstance with which I have been
    frequently struck. Mr. Procter and others have also remarked that the
    Aurora is generally formed in a sort of “mist or imperfect vapour.” The
    second, that Auroræ, or portions of them, are frequently near to the earth’s
    surface. Instances of this are given in the section on the Height of the
    Aurora, notably the experiences of Sir W. Grove and Mr. W. Ladd.</p>

    <div class="sidenote">Coincidence
    of Auroral
    lines with
    telluric solar
    lines.</div>

    <p>On this point, too, note the peculiarities of the red line, which (and, as I
    find, the green line also) are coincident with, or very close to, telluric bands or
    groups of lines in the solar spectrum usually attributed to moisture. (See
    Plate XIII. fig. 2.)</p>

    <div class="sidenote">Continuous
    spectrum.</div>

    <p>I think we may also claim the continuous spectrum in the Aurora in
    further proof of water-vapour (see Plate XV. spectrum 7). The continuous
    spectrum of the Aurora is also, to my observation, more local and dense in
    the spectroscope than the glow generally seen between the lines or bands
    in gas-spectra.</p>

    <div class="sidenote">Violet-pole
    spectrum
    discussed. Most spectra
    have a
    general as
    well as special
    character.</div>

    <p>B. The question of the violet-pole spectrum. Here I make the remark
    that in comparing other spectra with that of the Aurora, it is, I think, too
    much the practice to trust to the coincidence (more or less perfect) of one
    or perhaps two lines out of many; whereas we know by experience that
    most spectra have so well-marked a general as well as special character
    that, when once seen, they are recognized afterwards with the greatest ease
    and without measurements. An experience and proof of this is found in a set
    of “Photographed Spectra” which the Autotype Company have reproduced
    for me.</p>

    <div class="sidenote">Coincidence
    of one or
    two lines not
    sufficient to
    establish
    identity.</div>

    <p>Of course no two given spectra can be considered identical unless their
    principal lines coincide; but, on the other hand, the coincidence of one or
    two lines out of many, without other features, cannot be satisfactorily or conclusively
    held to establish identity.</p>

    <div class="sidenote">Ångström’s
    compared
    spectra.</div>

    <p>In Professor Herschel’s letter (Phil. Mag. ser. 4, vol. xlix. p. 71), Professor
    Ångström’s representation of the “spectrum of the glow discharge round the
    negative pole of air-vacuum tubes” is given, in comparison with the Aurora-lines
    and those of olefiant gas. This illustration is here introduced.</p>

    <p></p>

    <p>Ångström’s representation of the Spectrum of the glow discharge round the negative pole of Air-vacuum
    tubes, and its comparison with the Spectrum of the Aurora.</p>

    <div class="figcenter" style="width: 700px;">
    <!-- <img src="assets/aurorae/images/graph.jpg" width="700" height="200" alt="" /> -->
    <p class="caption">Wave-lengths, in hundred-thousandths of a millimetre.</p>
    </div>

    <p>It is unfortunate that in this illustration and in Professor Herschel’s paper
    the wave-lengths of the Aurora-lines are not given in figures, but must be
    roughly calculated from the scale. Professor Herschel speaks of Ångström’s
    drawing as representing a <i>normal</i> spectrum, and as derived from authentic
    sources, such as Vogel, Barker, and others; but beyond this we are not
    certain as to its origin.</p>

    <p>In illustration of the difficulty of constructing any thing like a general
    typical Aurora-spectrum I append a Table of eight Auroral spectra taken at
    hazard:—</p>

    <div class="sidenote">Table of
    compared
    Aurora.</div>

    <p class="center">Auroral lines and bands.</p>

    <table summary="8 auroral spectra compared" class="borders">
      <tr>
        <th>Observers.</th>
        <th>Red.</th>
        <th colspan="2">Yellow.</th>
        <th>Green.</th>
        <th>Blue.</th>
        <th colspan="2">Indigo.</th>
        <th colspan="2">Violet.</th>
      </tr>
      <tr>
        <td class="bl">Vogel, April 9, 1871</td>
        <td>6297</td>
        <td>5569</td>
        <td>5390</td>
        <td>5233</td>
        <td>5189</td>
        <td>5004</td>
        <td>—</td>
        <td>4694 to 4629</td>
        <td>—</td>
      </tr>
      <tr>
        <td class="bl">Barker, Nov. 9, 1871</td>
        <td>6230</td>
        <td>5620</td>
        <td>—</td>
        <td>—</td>
        <td>5170</td>
        <td>5020</td>
        <td>4820</td>
        <td>—</td>
        <td>—</td>
      </tr>
      <tr>
        <td class="bl">Barker, Oct. 14, 1873</td>
        <td>6300</td>
        <td>5550</td>
        <td>—</td>
        <td>5330 to 5200</td>
        <td>—</td>
        <td>5050 to 4990</td>
        <td>4930 to 4850</td>
        <td>4740 to 4670</td>
        <td>4310</td>
      </tr>
      <tr>
        <td class="bl nw">A. Clarke, junr., Oct. 24, 1870</td>
        <td></td>
        <td>5690</td>
        <td>—</td>
        <td>5320</td>
        <td>—</td>
        <td>—</td>
        <td>4850</td>
        <td>—</td>
        <td>4350</td>
      </tr>
      <tr>
        <td class="bl">Backhouse, 1873</td>
        <td>6060</td>
        <td>5660</td>
        <td>—</td>
        <td>—</td>
        <td>5165</td>
        <td>5015</td>
        <td>—</td>
        <td>4625</td>
        <td>4305</td>
      </tr>
      <tr>
        <td class="bl">Backhouse, Feb. 4, 1874</td>
        <td>*</td>
        <td>5570</td>
        <td>—</td>
        <td>—</td>
        <td>5180</td>
        <td>4980</td>
        <td>4830</td>
        <td>4640</td>
        <td>4320</td>
      </tr>
      <tr>
        <td class="bl">H. R. Procter, 1870</td>
        <td>*</td>
        <td>*</td>
        <td>—</td>
        <td>*</td>
        <td>—</td>
        <td>—</td>
        <td>—</td>
        <td>*</td>
        <td>*</td>
      </tr>
      <tr>
        <td class="bl bb">Lord Lindsay, 1870</td>
        <td class="bb">—</td>
        <td class="bb">*</td>
        <td class="bb">*</td>
        <td class="bb">*</td>
        <td class="bb">*</td>
        <td class="bb">—</td>
        <td class="bb">—</td>
        <td class="bb">—</td>
        <td class="bb">*</td>
      </tr>
    </table>

    <p class="center smaller">* Mr. Procter’s and Lord Lindsay’s lines had no wave-lengths.</p>

    <div class="sidenote">Ångström’s
    drawing
    discussed.</div>

    <p>On examining Ångström’s diagram it certainly seems to me that, upon
    the showing of the drawing itself, the coincidences are not very exact. All
    three of the violet-pole bands appear to be less refrangible than the Aurora-lines
    with which they are compared-the middle one (at 47) considerably
    so, the one near E (at about 52·30) appreciably so, and the third (at 43)
    slightly so.</p>

    <div class="sidenote">Diagram of
    Vogel’s
    Aurora and
    violet-pole
    spectrum.</div>

    <p>As it seemed desirable to adopt a specific Aurora-spectrum for comparison,
    and to show such comparison on a somewhat larger scale than Ångström’s
    drawing, I prepared the diagram shown on Plate XI. fig. 1. The upper
    spectrum is Vogel’s, already described and figured on Plate XIII. The lower
    spectrum is that of “Air, violet pole,” Plate XV. spectrum 2, graphically
    shown. I can only find one absolute coincidence in the two compared spectra
    in this diagram.</p>

    <p>It should, too, I think, be borne in mind that there is a great difference
    in the character of the compared spectra, whether as shown in Ångström’s
    drawing or mine—the bands of the violet-pole spectrum mostly degrading
    towards the violet, while the lines or bands of the Aurora in no way possess
    that character&nbsp;<span class="footnote">Ångström’s drawing, in giving this character to the two Aurora-bands which are said to
  correspond with violet-pole bands about 47 and 43, is incorrect, and calculated to mislead by
  giving the Aurora-bands a feature corresponding to the violet-pole bands which they do not
  possess. I am not aware of any Aurora-line or band which is described as distinguished by
  degrading towards the violet.</span>.</p>

    <div class="sidenote">Dr. Vogel’s
    violet-pole
    and Aurora-lines.</div>

    <p>To assist in the foregoing violet-pole comparison I add the following Table
    derived from Dr. Vogel’s memoir:—</p>

    <table summary="Vogel's table">
      <tr>
        <th colspan="2">Violet-pole lines.</th>
        <th colspan="2">Aurora-lines.</th>
      </tr>
      <tr>
        <th>W.L.</th>
        <th></th>
        <th>W.L.</th>
        <th></th>
      </tr>
      <tr>
        <td>6100,</td>
        <td rowspan="2" class="vm nw"> broad, moderately bright stripe</td>
        <td rowspan="2" class="vm">6297,</td>
        <td rowspan="2" class="vm">very bright stripe.</td>
      </tr>
      <tr>
        <td>5945,</td>
      </tr>
      <tr class="spaced">
        <td>5459,</td>
        <td rowspan="2" class="vm"> broad, moderately bright stripe </td>
        <td>5569,</td>
        <td>brightest line of spectrum.</td>
      </tr>
      <tr>
        <td>5289,</td>
        <td>5390,</td>
        <td>extremely faint line.</td>
      </tr>
      <tr class="spaced">
        <td>5224,</td>
        <td>very bright line</td>
        <td>5233,</td>
        <td>moderately bright.</td>
      </tr>
      <tr>
        <td>5147,</td>
        <td>faint line</td>
        <td>5189,</td>
        <td>moderately bright.</td>
      </tr>
      <tr>
        <td>5004,</td>
        <td>bright line</td>
        <td>5004,</td>
        <td>very bright line.</td>
      </tr>
      <tr>
        <td>4912,</td>
        <td>fainter than last.</td>
        <td></td>
        <td></td>
      </tr>
      <tr class="spaced">
        <td>4808,</td>
        <td>very faint line.</td>
        <td>6694,</td>
        <td rowspan="3" class="vm nw">band less brilliant in the middle.</td>
      </tr>
      <tr>
        <td>4704,</td>
        <td>very intense line.</td>
        <td>4663,</td>
      </tr>
      <tr>
        <td>4646,</td>
        <td>very faint line.</td>
        <td>4629,</td>
      </tr>
      <tr class="spaced">
        <td>4569,</td>
        <td>moderately bright.</td>
        <td></td>
        <td></td>
      </tr>
      <tr>
        <td>4486,</td>
        <td>moderately bright.</td>
        <td></td>
        <td></td>
      </tr>
      <tr>
        <td>4417,</td>
        <td>quite faint line.</td>
        <td></td>
        <td></td>
      </tr>
      <tr>
        <td>4346,</td>
        <td>moderately bright line.</td>
        <td></td>
        <td></td>
      </tr>
      <tr>
        <td>4275,</td>
        <td>very bright line.</td>
        <td></td>
        <td></td>
      </tr>
    </table>

    <p></p>

    <p>On examination of these figures it will be seen that 5224 and 5233 are
    fairly close, and that 5004 is coincident. Beyond these there is little to
    identify the spectra.</p>

    <div class="sidenote">Conclusions
    arrived at
    adverse to
    the violet-pole
    theory.</div>

    <p>As the general result of my observations and a comparison of the foregoing
    spectra and tables, I see no reason for giving to the violet-pole glow
    any special or distinguished place in a comparison with the Aurora, and certainly
    not for assigning to it the nearly absolute monopoly of the spectrum.
    It is true that the line γ in the violet-pole glow (Plate XV. spectrum 2),
    which, by the way, degrades towards the red, is in close coincidence with one
    of the Aurora-lines; but how are the brighter bands α and β accounted for?
    These, as I have before pointed out, alone survive when the tube is placed at
    a distance from the slit. It is true they are thus reduced to shaded-off lines
    in lieu of bands; but the difficulty still remains, that they are conspicuous
    for their absence in the Aurora-spectrum. On the whole, I cannot but
    conclude that Professor Ångström’s theory fails. At all events, if the violet-pole
    glow-spectrum is to represent the Aurora-spectrum, it must be under
    conditions different from those by which it obtains in dry-air vacuum-tubes
    or flasks at ordinary temperature.</p>

    <div class="sidenote">Phosphorescence
    or
    fluorescence
    of the yellow-green
    line.</div>

    <p>4th. I feel more in accord with Professor Ångström’s memoir upon the
    subject of the phosphorescence or fluorescence of the bright yellow-green
    Aurora-line.</p>

    <div class="sidenote">External
    features of
    Auroræ confirmatory
    of
    this.</div>

    <p>I do not notice that the Professor touches upon the external features of the
    Aurora in respect of this question.</p>

    <p>October 20, 1870.—I noted the grand Auroral display of that evening, including
    “streamers of opaque-white <i>phosphorescent</i> cloud very different from
    the more common transparent Auroral diverging streams of light.”</p>

    <p>February 4, 1872.—A fine display. The first signs were (in dull daylight)
    “a lurid tinge upon the clouds, which suggested the reflection of a distant
    fire, while, scattered among these, torn and broken masses of white vapour,
    <i>having a phosphorescent appearance</i>, reminded me of a similar appearance
    in October 1870.” (Other instances of this effect will be found in the section
    Aurora and Phosphorescence.) Day Auroræ, too, we might suppose could
    hardly be seen without the presence of some phosphorescent glow.</p>

    <div class="sidenote">Other confirmatory
    circumstances. Conclusion
    in favour of
    the theory.</div>

    <p>Having regard to the near proximity of the phosphoretted-hydrogen band
    to the bright Aurora-line, to the circumstance of this band brightening by
    reduction of temperature (a phenomenon probably connected with ozone), to
    the peculiar brightening of one line in the green in the “Aurora” and “phosphorescent”
    tubes (the phosphorescent tubes probably containing O), and
    to the observed circumstance that the electric discharge has a phosphorescent
    or fluorescent after-glow (isolated, I believe, by Faraday), I feel there is strong
    evidence in favour of such an origin to the principal Aurora-line, if not to the
    red line as well.</p>

    <div class="sidenote">Invariability
    of gas-spectra
    questioned.</div>

    <p>5th. Professor Ångström opens a wide door to discussion in his proposition
    of the invariability of gas-spectra, and I do not now attempt to follow in
    detail this interesting part of the present subject. Suffice it to say, that if the
    Professor lays down this proposition in its strictest sense (I can hardly suppose
    he so meant it), there is, so far as I am aware, no one spectrum that can
    at all claim comparison with the Aurora-spectrum. Giving greater latitude
    to the Professor’s words, I reply, upon competent authority, that lines vary
    in number and brilliancy with temperature, and in breadth with pressure.
    Kirchhoff, too, in speaking of vapour-films as increasing the intensity of lines,
    states “it may happen that the spectrum appears to be totally changed when
    the mass of vapour is altered.” We may, too, now add magnetism as capable
    of effecting a change in certain spectra, not only as to brilliancy, but even as
    to position of lines. (Chautard’s Researches, ‘Philosophical Magazine,’ 4th
    series, vol. 1. p. 77, and experiments detailed in Chap. III. of this work.)</p>

  </section>

  <!-- Chapitre 13_________________________________________________________-->
  <section class="chapter" id="chapter-13">
    <h3 class="titlechapter" id="chap-13">The Oxygen-spectrum in relation to the Aurora (Procter and Schuster)</h3>
    <p class="shorter">The Oxygen-spectrum in relation to the Aurora</p>

    <div class="sidenote">Procter’s
    oxygen-spectrum.</div>

    <p>In a communication to ‘Nature,’ Mr. H. R. Procter has pointed out an
    apparent coincidence in position of several of the Auroral lines with those of
    a spectrum occasionally obtained from air at low pressure with a feeble
    discharge. It is, he says, sometimes exhibited in lumière (phosphorescent?)
    tubes, and he believed it, in part at least, to be the spectrum described by
    Wüllner (Philosophical Magazine, June 1869) as a new spectrum of oxygen.</p>

    <span class="sidenote">How obtained.</span>

    <p>He had obtained it very vividly in pure electrolyzed oxygen with a
    feeble discharge, but some perplexing observations made him doubtful of
    its origin.</p>

    <p>Plate XI. fig. 4 gives a representation of this spectrum as shown by
    Mr. Procter, except that my drawing is in black for white.</p>

    <div class="sidenote">Compared
    spectra described.</div>

    <p>The upper spectrum is that above mentioned, the centre one that of the
    Aurora, the lower one the lines of Na and H for comparison. The Auroral
    yellow-green line, in January 1870, was found by Mr. Procter coincident with
    a bright line or band in the tube (with a spectroscope of a 60° bisulphide
    prism, and magnifying-power about six). The third and fifth lines in the
    Aurora seemed also to correspond with tube-lines. As to these Mr. Procter
    says they were not bright enough to be compared with the same accuracy as
    the yellow-green line, but that the positions could not be far wrong.</p>

    <div class="sidenote">Mr. Procter’s
    subsequent
    views. Yellow-green
    line
    traced to
    some form
    of hydrocarbon.</div>

    <p>Mr. Procter subsequently (‘Edinburgh Encyclopædia,’ art. “Aurora”)
    considered he traced the yellow-green tube-line to some form of hydrocarbon.
    On examination with instruments of greater dispersion, it was found that,
    though more refrangible than the first band of citron acetylene (candle-flame),
    it was less so than the Aurora-line. The tube-band, too, was shaded towards
    the violet, which was not the case with the Aurora-line.</p>

    <p>The question as between hydrocarbon and oxygen I did not then consider
    as disposed of. With the lumière tubes the question might be open, but I did
    not see how it could be in the case of the electrolyzed oxygen-spectrum.</p>

    <p>From a comparison of the tube-spectra, I have shown that although the
    spectra of the carbon and oxygen tubes are proved to be, photographically,
    as a whole, distinct, they have, as to position of some of the principal lines
    in the central part of the spectrum, a very close resemblance.</p>

    <div class="sidenote">Probability
    that O may
    play a part
    in the
    Aurora-spectrum.</div>

    <p>That oxygen may in some form play a part in the Aurora seems highly
    probable; how far it is spectroscopically detected seems a different question.</p>

    <div class="sidenote">Difference
    between air-spark
    and
    tube-spectra.</div>

    <p>Ångström and Herschel suggest its presence in the Aurora in connexion
    with phosphorescence or fluorescence. With a spark-discharge in air at ordinary
    pressure, a mixed spectrum of bright lines of N and O is found; while
    in the case of Geissler vacuum-tubes (representing a glow-discharge in a
    much more rarefied atmosphere) the N lines appear mainly to usurp the
    spectrum.</p>

    <div class="sidenote">H₂O tube
    referred to.</div>

    <p>It must, however, be borne in mind that a Geissler tube, as to temperature
    at least, in no way represents the conditions of the Aurora; and when we
    remember the association of oxygen and ozone, and the way in which the
    latter is affected by heat, it may well be that temperature plays an important
    part in the matter. In proof of this conduct of oxygen, it may be cited that,
    in the case of a H₂O tube, the H lines come out sharp and brilliant in the
    spectrum, while what is seen of the O lines is comparatively weak, misty, and
    ill-defined. Vogel, it will be remembered, makes 5189 of the Aurora coincident
    with an O line.</p>

    <div class="sidenote">Residual
    phosphorescence
    in
    Geissler
    tubes. Garland
    tube.</div>

    <p>Professor Herschel has pointed out, and I have confirmed, that the residual
    phosphorescence in Geissler tubes, after the spark has passed, is probably
    associated with oxygen. He also alludes to the fact that when one of the
    globes of a “Garland” tube was heated, it did not shine after the spark had
    passed, apparently because of the destruction of the ozone by heat.</p>

    <p>[Some experiments with a tube of this description will be found detailed
    in Part III. Oxygen was not, I think, the gas it was filled with.]</p>

    <div class="sidenote">Dr. Schuster’s
    tubes
    described.</div>

    <p>Subsequently to my examination and comparison of the O and CO₂ spectra
    before detailed, Dr. Arthur Schuster was good enough to send me three
    vacuum-tubes of his own preparation, showing an oxygen-spectrum.</p>

    <p>One, with large disk-shaped brass electrodes, was unfortunately broken in
    transit. Dr. Schuster informed me it showed the carbonic-oxide spectrum as
    well as that of oxygen. The other two tubes had aluminium electrodes. They
    were similar in shape to ordinary Geissler tubes, but had attached to each a
    supplemental bulb containing dry oxide of manganese. Illuminated by the
    larger coil, one of these tubes (which had a slight crack in the manganese
    bulb) lighted up faintly; the other was fairly bright, and the glow had a
    somewhat reddish tint.</p>

    <p>Plate XVIII. fig. 15 represents as the upper spectrum Vogel’s Aurora,
    with W.L. numbers, as the middle spectrum the capillary part of Dr.
    Schuster’s O tube, and as the lower spectrum the negative (violet) pole of the
    same tube.</p>

    <span class="sidenote">Spectra described.</span>

    <p>The tube-spectra were mapped out with the aid of the diaphragm micrometer
    before described.</p>

    <span class="sidenote">Capillary.</span>

    <p>The capillary spectrum was mainly distinguished by four bright sharp
    lines—one in the red, between the red Aurora-line and D, two in the green,
    but considerably more refrangible than the yellow-green Aurora-line, while
    the fourth was found to be hydrogen F. The other lines in the spectrum
    were considerably fainter, and misty and band-like. The red line, though
    not brilliant, was fairly bright and sharp.</p>

    <p>The place of the less refrangible of the two bright bands in the violet-pole
    spectrum was occupied in the capillary spectrum by a faint glow only.</p>

    <span class="sidenote">Violet-pole.</span>

    <p>The violet-pole spectrum was recognized by two very bright broad bands
    of light in the green, each including within its limits one of the Aurora-lines.
    The bright red line in the capillary had a faint representative in the violet-pole
    spectrum, as also had the two bright lines in the green. Other fainter
    lines appeared in the blue, and three fairly bright ones towards the violet.</p>

    <div class="sidenote">Dr. Schuster’s
    remarks
    on
    the spectra.</div>

    <p>Dr. Schuster remarks that one of these O bright bands is closely coincident
    with a band in the CO spectrum, but that the CO band is bright towards
    one edge and fades off gradually thence, while the O band is of pretty
    uniform strength throughout. Dr. Schuster finds the wave-lengths of the
    violet-pole O bands to be as follows:—</p>

    <table summary="Schuster's results">
      <tr>
        <td>5205·0</td>
        <td rowspan="2" class="vm"><span class="x2">}</span> Brightest part 5255.</td>
      </tr>
      <tr>
        <td>5292·5</td>
      </tr>
      <tr>
        <td>5552·8</td>
        <td rowspan="2" class="vm"><span class="x2">}</span> Brightest part 5586.</td>
      </tr>
      <tr>
        <td>5629·6</td>
      </tr>
    </table>

    <div class="sidenote">His tubes
    free from
    impurity.</div>

    <p>He also gives as weak bands 5840-5900 and 5969-6010. Dr. Schuster
    comes to the conclusion that the green line of the Aurora is not due to
    oxygen, as, under considerable dispersion and with good definition, the
    oxygen-bands can be broken up into a series of lines, when the brightest part
    is found to lie at 5586, which is too much towards the red to compare with
    the Aurora-line. He notices that the more refrangible of the O bands
    corresponds with a line sometimes seen in the Aurora (Vogel’s 5233). The
    same remark will, however, apply to this last as to the other coincidence,
    viz., that a broad band can hardly represent a line—at least, the line can
    only be said to coincide in a loose and indefinite way. It is evident that
    Dr. Schuster’s tubes were free from what must now be considered an impurity
    in those examined by me and by Dr. Vogel, and that Mr. Procter’s suspicions
    of carbon impurities in these, and the ordinary oxygen-tubes, are thereby quite
    confirmed.</p>

    <div class="sidenote">Experiments
    with
    an open
    Geissler
    tube.</div>

    <p>In some experiments which we made (after receiving Dr. Schuster’s tubes)
    with an open Geissler tube, so arranged as to connect with an air-pump and
    gas-receiver, and thus from time to time to wash out the tube and vary its
    contents, we found the same impure spectrum as in the case of the sealed O
    tubes; and it seems to require a very large amount of precaution to avoid
    these impurities.</p>

    <div class="sidenote">Spectra of
    Dr. Schuster’s
    O tube
    examined.</div>

    <p>Dr. Schuster was kind enough to examine the spectra I mapped out, and
    which are shown in Plate XVIII. fig. 15, with the following results:—The
    lines Oα, Oβ, Oγ are those he has referred to under that designation in his
    communications to ‘Nature,’ and undoubtedly belong to oxygen. The bands
    A, B, and C are the bands characteristic of the negative pole. He finds A
    divided into two parts by a dark space. The spectrum of the negative pole,
    under good exhaustion, stretches into the capillary part; hence B appears in
    the capillary as a faint band. A similar thing happens with nitrogen. I., II.,
    III., and possibly 8 and 9, he thinks, are due to the spark-spectrum of oxygen,
    obtained when the jar and a break are interposed, the brighter lines of the
    line-spectrum being always present at the negative pole. These last-mentioned
    lines I have already referred to, as having been found by me in a tube
    showing phosphorescence after the spark has passed. (Compare Plate XVIII.
    fig. 15, O violet pole, with Plate XV. spectrum 5.) Nos. 1 and 2, he thinks,
    are due to some foreign matter, as they are not in all his tubes.</p>

    <p>Dr. Schuster often finds that a spectrum due to the aluminium electrodes
    is seen in tubes under great exhaustion; and this he considers is the spectrum
    of aluminium oxide. A drawing of this spectrum is found in Watts’s ‘Index
    of Spectra,’ plate iii., “Aluminium first Spectrum.” To this, he thinks, are
    also due the bands, or sets of lines in my aluminium-arc spectrum (‘Photographed
    Spectra,’ plate ii.), and he believes lines 3, 4, 5, 6, and 7 in the
    mapped-out spectra are due to it. It would thus appear that the lines due
    to O are few in number, and do not well compare with the Aurora-spectrum.</p>

    <p></p>

    <hr class="chap" />

    <p></p>

  </section>

  <!-- PART III_________________________________________________________ -->
  <section class="part">
    <h2  class="title-part" id="part-3">Magneto-electric experiments in connexion with the aurora</h2>
    <p class="shorter">Magneto-electric experiments in connexion with the aurora</p>

    <h3>Introduction</h3>

    <div class="sidenote">Object of
    experiments. Description
    of apparatus
    employed. Electro-magnet. Battery.</div>

    <p>The set of experiments detailed in Chapters XIV. to XIX. was mainly conducted
    for the purpose of testing, in connexion with the Aurora, the action of
    a magnet upon the electric glow <i>in vacuo</i> and on the spark at ordinary pressure.
    It also includes some observations on the glow from the violet pole with and
    without the magnet, and on the glow obtained from one wire only. The
    apparatus employed was a Ladd’s electro-magnet, with poles 10¼ inches high
    by 2 inches across, each pole being surrounded by a movable helix, composed
    of two sets of stout copper wire wound together, so that they could
    be used either in one length or as independent coils excited at the same time.
    The latter form of arrangement was employed by us. In most of the experiments
    conical armatures were employed for the purpose of bringing the action
    of the poles to bear upon the subjects examined. A contact-maker was
    added to the magnet, so that it could be put rapidly in or out of action
    without disturbing the wires. The battery used to excite the magnet was of
    the form known as that of Dr. Huggins, and consisted of four vulcanite cells
    in a frame, each holding seven pints of bichromate solution, and containing
    two carbon and one zinc plate, each 13½ by 6 inches.</p>

    <div class="sidenote">Small coils. Larger coil. Magnetic
    curves obtained.</div>

    <p>A winch and pulley enabled the whole set of plates to be lowered into the
    liquid and withdrawn at pleasure, and the large quantity of solution gave the
    battery a considerable amount of constancy. We found it could be used
    for two evenings’ work, of four hours each, without any material dropping in
    power. For obtaining the glow in the Geissler and other small tubes, a
    Ruhmkorff coil, giving a ½-inch spark, excited by one plate of a ½-gallon
    bichromate (bottle form), was used. For the glow in the larger tubes and
    the spark in air a larger coil, giving a two-to three-inch spark, and worked by
    two ½-gallon double-plate bichromates, was employed. Notes were taken
    of the experiments, and drawings of the effects at the time; and these
    are reproduced almost literally in the text and Plates comprised in
    this Part. To ascertain the direction and extent of the magnetic curves,
    we covered large sheets of cardboard, placed over the poles, with iron
    filings; excited the magnet so as to obtain the curves, and then obtained
    permanent prints from the filings by spraying the cardboard with tannin
    solution. The magnetic effects were thus found to extend to a radius of
    at least ten inches (see diagram, Plate XVII. fig. 1, showing magnet-poles
    and curves on a ¼ scale).</p>

    <div class="sidenote">Chautard’s
    investigations
    kept
    in view. Evidence
    obtained of
    change in
    colour, form,
    &amp;c. of
    Aurora. Ångström’s
    flask-experiment
    tried.</div>

    <p>In the vacuum-tube experiments we held Mons. J. Chautard’s investigations
    (on the action of magnets on rarefied gases in capillary tubes rendered
    luminous by the induced current, Phil. Mag. 4th series, vol. 1. p. 77) in view.
    We obtained in our experiments plenty of evidence of a change of colour and
    form in the discharge under the magnetic influence; and both simple and
    compound spectra were found to be much varied by the exaltation or suppression
    of some parts of the spectrum, so that apparently new lines sprang
    up; but we failed to trace actual change of position or wave-length in any
    given line, though we carefully looked for it. A portion of our researches
    was directed to the subject of Ångström’s experiment of filling a dry flask
    with a violet glow, analogous to that from the negative pole. We entirely
    failed in obtaining the same result while two wires and an uninterrupted
    circuit were employed. When, however, we attached a negative wire only
    (the other wire being left free) to an exhausted globular receiver, we obtained
    an effect very similar to that referred to in Prof. Ångström’s memoir.</p>

    <div class="sidenote">General
    results of
    experiments. Bulb effects
    noticed as a
    mode of
    analysis of
    gases.</div>

    <p>The general result of the experiments was to prove, assuming the Aurora
    to be an electric discharge, the great influence the magnetic forces may
    exercise on the colours, form, motions, and probably the spectrum also
    of that phenomenon. It is easy to conceive that the variation in number, and
    intensity of the lines which has been remarked in Auroral spectra may have
    its origin in such a cause. The influence of the magnet on the capillary
    stream was mainly in colour and intensity; but in the bulbs the effects were
    still more marked and striking, and, in a greater or less degree, different in
    the case of each gas which we examined. A careful and extended study of
    these effects, conjointly with the changes in the spectrum, might possibly
    form a new and valuable mode of analysis of compound gases. This is well
    illustrated in the case of the iodine and sulphur tubes which we examined.</p>

    <p></p>

    <figure class="plate" id="plate17">
      <!-- <img src="assets/aurorae/images/plate17.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    </section>

  <!-- Chapitre 14_________________________________________________________ -->
  <section class="chapter" id="chapter-14">
    <h3 class="titlechapter" id="chap-14">Examination of Geissler-tubes under action of the Magnet</h3>
    <p class="shorter">Examination of Geissler-tubes under action of the Magnet</p>

    <h4 id="chap-14-1"><i>Nitrogen-tubes.</i></h4>

    <div class="sidenote">Nitrogen-tube
    No. 1. Discharge
    described. Spectrum
    described. Capillary
    stream. Positive
    bulb. Violet-pole
    glow.</div>

    <p>(1) A small Geissler tube (No. 1) was lighted up by the small coil. The capillary
    part showed a very bright, slightly rosy-tinted stream. Negative bulb was
    filled with rosy-purple light, the violet-pole glow being confined to the extent of
    the electrode. Positive bulb of the same rosy-purple colour, but stream slightly
    contracted in volume. Glow throughout quiescent, and no stratification in
    the tube. A compound-prism spectroscope, taking in the whole of the spectrum,
    showed in the capillary stream, from yellow to red, a fairly bright wedge,
    having a dark band in the centre, and six bright columns, with dark lines at
    intervals, shading off on either side. On the more refrangible side of the
    yellow, the spectrum was composed of a set of bright bands and lines in the
    green, blue, and purple, one line only (in the green) standing out very bright.
    In the yellow and red no bright line stood out alone. The positive bulb gave
    a fainter spectrum of the same character, mainly confined to the centre, the
    violet, yellow, and red not being well seen. When the violet-pole glow was
    examined, the general character of the spectrum was quite changed: a brilliant
    broad band in the violet, a bright narrower one in the blue, and two
    bright lines in the green, with intermediate fainter lines throughout, were the
    main features. The yellow and red part of the spectrum was also changed.
    The yellow was fairly and evenly distinct up to the dark band; then came a
    somewhat brighter orange band, and after that the red, but rather obscure
    and cut off. No absolutely bright line could be traced in the red.</p>

    <div class="sidenote">Nitrogen-tube
    No. 2. Glow described. Difference
    of spectra of
    capillary
    stream and
    violet-pole
    glow. Junction of
    the violet-pole
    glow
    and capillary
    stream.</div>

    <p>(2) To compare the capillary stream and the violet-glow, a second nitrogen-tube
    (No. 2) was used. This tube was larger in bulk and bore than No. 1.
    The glow in the bulbs was considerably fainter and more salmon-coloured;
    and there was much stratification in both, extending to the capillary bore.
    (This stratification was considered due to H, as the three principal lines of
    that gas came out very brightly in the spectrum.) The difference of the
    spectra of the capillary stream and of the violet-pole glow was extremely well
    marked—the former consisting of a set of bright lines and bands of fairly
    uniform intensity, while the latter was split up into a few bright bands with
    fainter lines between. The yellow and red of the violet-glow were very weak
    as compared with the same region of the capillary spectrum. No bright line
    appeared in the red. The tube being properly adjusted for the purpose, the
    junction of the violet-pole glow and the capillary red-glow was easily observed.
    The bright bands of the violet-pole were seen to run into the
    capillary line-spectrum, and then, gradually getting finer and more pointed,
    to fade out.</p>

    <div class="sidenote">Tube No. 1
    between the
    poles of the
    magnet. Change of
    colour in
    capillary
    stream. “Tailing-over”
    of
    capillary
    stream.</div>

    <p>(3) The capillary part of tube No. 1 was arranged between the poles of
    Ladd’s electro-magnet, the conical ends of the armatures almost touching the
    tube (Plate XVII. fig. 1). With the magnet not excited, the capillary stream
    was bright and of a slightly rosy-yellow tinge. It varied a little in apparent
    diameter with the current. As soon as the magnet was excited the capillary
    stream, as also (in a less degree) that in the bulbs, were seen to contract, and
    to change from a <i>rosy</i> tint to a distinctly <i>blue-violet</i>. The polished armatures,
    acting as reflectors, showed this change of tint in a most marked manner each
    time the magnet was excited. At the same time the capillary stream was
    seen to run into the negative bulb, as if overflowing, and with an effect resembling
    the “tailing-over” of a gas-flame. This effect took place each time the
    magnet was excited, and was not found at the positive-bulb end.</p>

    <div class="sidenote">Spectrum
    examined.</div>

    <p>Occasionally, when the magnet was excited, flashes of light were discharged
    in the negative bulb from the capillary towards the violet-pole. The
    spectrum was then carefully examined. No change was seen in the actual
    position of any of the lines or bands when the tube was influenced by the
    magnet, but those towards the violet end of the spectrum were conspicuously
    brightened.</p>

    <div class="sidenote">Negative
    bulb between
    poles
    of the magnet. Positive
    bulb within
    action of
    the magnet.</div>

    <p>(4) The extremity of the negative bulb was now placed between the poles
    of the magnet. A bright violet-coloured arc, following the magnetic curve,
    was at once formed, as in the case of the large Plücker tubes; and at the
    same time a straight stream of not very bright light ran along the bulb. The
    positive bulb was next placed within the action of the magnet; and immediately
    a brilliant spiral of flickering light appeared in the bulb, lighting it
    up, and reminding one in shape of the spiral which water forms on being
    poured from a lipped jug (see Plate XVII. fig. 9).</p>

    <div class="sidenote">Spiral
    formed.</div>

    <p>This was repeated each time the magnet was excited. The spiral, though
    flickering in character, was permanent in form, and inclined to the side of the
    tube which was in contact with the N pole of the magnet.</p>

    <p></p>

    <h4 id="chap-14-2"><i>Oxygen-tubes.</i></h4>

    <div class="sidenote">O tube No.
    1; spectrum
    described.</div>

    <p>A tube (No. 1) was lighted up and examined with the spectroscope, and
    found to give the spectrum shown on Plate XIV. spectrum 3, but with a
    strong set of H lines in addition.</p>

    <div class="sidenote">O tube No.
    2; spectrum
    described.</div>

    <p>A second tube (No. 2) was then lighted up. The spectrum was a bright
    one, similar to the foregoing, the principal H lines being present, but not
    strong.</p>

    <div class="sidenote">Tube-glow
    described. Effect upon
    glow when
    magnet
    excited. Bulbs between
    poles
    of the magnet. Effect of
    magnet on
    spectrum.</div>

    <p>The red region was indistinct, and showed no prominently bright line. The
    bulbs were mainly of a slightly blue-grey tint, with a steady glow. Capillary
    stream quite pale white, with a very slight tinge of red. Violet-glow small
    and confined to the electrode. Upon the magnet being excited, the capillary
    stream became intensely brighter, and the glow in both bulbs contracted into
    a single bright stream, which curved towards the sides of the bulbs at right
    angles to the magnetic poles, and changed from side to side with the current.
    This effect was very marked, and was more apparent in the positive than the
    negative pole. A faint stratification was seen in both bulbs. Upon either
    bulb being placed between the armatures, the glow left the electrode point
    and condensed into one bright stream, running along the side of the tube and
    curving at each end (Plate XVII. fig. 10). No trace whatever of tendency
    to form a spiral was seen. The spectrum with the magnet on was very conspicuously
    brightened up throughout. A set of fluted bands with a bright
    line among them appeared in the red, and several lines or bands appeared
    in the violet which could not be seen before. The bright red line, upon
    measurement, proved to be the hydrogen-line C. It thus seemed brighter in
    proportion than the F line, although, with the magnet off, the latter was well
    seen, while the C was not. No actual change in position of the spectrum-lines
    could be detected.</p>

    <p>[It is to be noticed that the O tubes employed were those used by me in
    former experiments, and had the bright lines now attributed to hydrocarbon
    impurity. Their bulb-effects differed, however, entirely from those of the CO₂
    tube. (Compare figs. 10 and 11, Plate XVII.)]</p>

    <h4 id="chap-14-3"><i>Hydrogen-tubes.</i></h4>

    <div class="sidenote">H tube, No.
    1; glow described.</div>

    <p>A small H Geissler tube (No. 1) was selected, and lighted up by the small
    coil. The capillary was a bright white-pink stream, with a tendency to redden
    at times. The bulbs were both of a faint blue-grey tint, with coarse lenticular
    stratification. The violet-pole glow was pale and white as compared with
    that of N.</p>

    <div class="sidenote">When the
    magnet was
    excited,
    whole character
    of
    tube
    changed. Unexcited
    spectrum
    described. Effect when
    magnet was
    excited.</div>

    <p>When the magnet was excited, the whole character of the tube changed.
    The capillary stream diminished in brightness and in apparent volume, and
    changed to a deep amber-yellow. The bulbs lost some of their light, and their
    coarse stratification; being, in lieu, filled with a vertical condensed stream of
    moderate light, in which a fine stratification only was seen. The stream in
    the positive bulb had a tendency to the spiral form. The capillary, each time
    the magnet was excited, “tailed over” into the negative bulb, as in the case
    of N, looking as if it were squeezed out of the capillary bore. The unexcited
    spectrum was found to consist of the usual principal lines of H on a continuous
    glow, with the intermediate bands and finer lines, which are usually suspected
    to be due to impurity. The sodium-line was also seen. When the magnet
    was excited, the spectrum grew much fainter—the continuous glow in the red
    and blue, and the red and blue lines, nearly disappearing, and the line in the
    green alone shining out conspicuously. No change of place in the lines could
    be noticed.</p>

    <div class="sidenote">No. 2 H
    tube; effects
    described.</div>

    <p>A longer H tube (No. 2) was then tried, with similar effects, except that
    the diminution in brightness was not so conspicuous. When the negative bulb
    of the tube No. 1 was placed between the poles of the magnet, a stream of
    light was formed, and the stratification became finer. The same effect took
    place with the positive bulb, with a tendency to the spiral form.</p>

    <h4 id="chap-14-4"><i>Water-Gas (H₂O) tube.</i></h4>


    <p><span class="sidenote">Water-gas (H₂O) tube; effects produced described.</span>A faint purple glow was seen in each bulb, the tube not lighting-up brightly.
    The capillary showed a slightly rosy-tinted, grey stream of brighter light.
    With the magnet on, the glow in the bulbs was condensed into a single bright
    stream. The capillary brightened up, and assumed a yellow tint—this effect
    being principally confined to that portion which was between the conical ends
    of the armatures, and gradually diminishing as the distance increased from
    these. Without the magnet, the principal H lines showed brightly in the
    spectrum, the O lines being misty and indistinct. With the magnet on, the
    O lines and spectrum generally brightened up.</p>

    <h4 id="chap-14-5"><i>Ammonia-tube.</i></h4>

    <div class="sidenote">Ammonia-tube;
    lighting-up
    described. Spectrum
    described.</div>

    <p>This tube was difficult to light up. Hardly any light was seen in the bulbs,
    except a very faint purple glow at the electrodes. In the capillary part a
    fairly bright stream of purple-white light appeared. The spectrum was a
    faintly shown one of N and H. The effect of the magnet was to reduce the
    brightness of the glow in the capillary, but with little marked action on the
    bulbs, except to condense the faint glow into a slightly bright stream running
    along the side of the tube.</p>

    <p>On a subsequent examination the tube and spectrum both brightened up
    under the influence of the magnet. The N lines, which were faint without
    the magnet, shone out under its influence distinctly—the red and yellow
    parts of the spectrum specially showing this effect. The H lines also
    brightened up, but hardly so much in proportion as the N.</p>

    <h4 id="chap-14-6"><i>Carbonic-Acid tube.</i></h4>

    <div class="sidenote">Tube
    marked
    C A; lighting-up
    described.</div>

    <p>A Geissler tube marked C A was examined. Capillary stream a brilliant
    bluish white; bulbs grey-blue, with a slight tint of green; slight stratification
    in positive bulb; stream diffuse, not quite filling the bulbs, and changing in
    volume as the coil-break was touched; glow round the violet-pole considerable,
    but markedly white in tint, rather than violet; stratification strong in
    capillary. With magnet excited, the capillary stream diminished in volume,
    but greatly increased in brightness. It “tailed over” into the negative bulb,
    and the stream through both bulbs curved towards the sides. A slight
    pattering noise was heard in the tube. In the positive bulb bright, imperfectly
    formed, saddle-shaped rings of light, with a tendency to spiral formation,
    were seen, somewhat similar to the effects in the Plücker tube after described
    (see Plate XVII. fig. 11).</p>

    <div class="sidenote">Effects
    when magnet
    was
    excited.</div>

    <p>The whole spectrum, under influence of the magnet, became much brightened
    up. Faint bands in the red came out bright, as also did some in the violet.
    The violet-glow was examined (without the magnet), and the light was found
    condensed into four prominent shaded bands, one red, one yellow-green, one
    green, and one blue, with fainter bands seen between.</p>

    <h4 id="chap-14-7"><i>Chlorine-tubes.</i></h4>

    <div class="sidenote">Chlorine-tube
    No. 1
    lighted-up. Action of
    magnet
    upon the
    tube and
    spectrum.</div>

    <p>A chlorine-tube (No. 1) was lighted-up with the small-coil. Capillary stream
    of a pale green tint. Bulbs with very little glow in them; spectrum pale, and
    not very distinct. Under action of the magnet this tube brightened up
    throughout, and the glow became more condensed, and ran to the sides of the
    tube. The spectrum also brightened, the faint lines becoming stronger, but
    the general character was preserved.</p>

    <div class="sidenote">Chlorine-tube
    No. 2
    lighted-up. Effect on
    glow when
    magnet was
    put on.</div>

    <p>A second chlorine-tube (No. 2) was then tried. Both bulbs were completely
    filled with a dense white (very slightly rosy-tinted) opaque light, and capillary
    the same, but brighter. A very slight violet tinge was seen at the negative
    pole. When the magnet was put on, both bulbs were at once filled with
    flickering bright streams of light, running towards the side of the tube,
    according to the direction of the current.</p>

    <p>The capillary stream at the same time changed from white to an intense
    bright green. The spectrum without the magnet consisted of sets of lines,
    with two well-marked absorption-spaces between, all seen somewhat faintly,
    as if through a mist.</p>

    <div class="sidenote">Changes in
    spectrum
    when magnet
    was
    excited.</div>

    <p>When the magnet was put on, the marked character of the absorption
    spaces was lost. The sets of lines in the yellow-green and green started up
    intensely bright, while those in the blue only slightly brightened.</p>

    <p>The misty appearance was altogether lost, and the bright lines all shone up
    upon a perfectly dark background, with a strikingly metallic look; we could
    not, however, trace change of position or actually new lines. It seemed as if
    lines which had been faint in the yellow-green and green region suddenly
    increased in intensity, the other parts of the spectrum not being similarly
    influenced. They quite flashed up when sudden contact was made with the
    magnet commutator.</p>

    <h4 id="chap-14-8"><i>Iodine-tubes.</i></h4>

    <div class="sidenote">Iodine-tube
    No. 1.</div>

    <p>This tube (No. 1) had been used for photographic purposes, and the bulbs
    were partly obscured by a white deposit.</p>

    <div class="sidenote">Lighting-up
    described. Effect of
    touching
    one wire
    with the
    finger.</div>

    <p>On lighting it up, both bulbs were filled with a violet-grey diffused light,
    with much coarse well-marked lenticular stratification. This stratification was
    mainly lost on changing the direction of the current, but made its reappearance
    when one conducting-wire was touched with a finger. This effect was
    still more marked when one finger of each hand was applied to the wire.
    The capillary stream was of a pale lemon-yellow. On putting on the magnet
    the light in the whole tube was nearly extinguished, a faint thin stream of
    condensed light running through the centre of the tube alone remaining.</p>

    <div class="sidenote">Effect of the
    magnet.</div>

    <p>On placing the bulbs between the magnet-poles, effects were produced
    similar to those in the case of the tube, after described (p. 144, and marked
    Si Fl₆), but in a less marked degree.</p>

    <div class="sidenote">Tube again
    tested.</div>

    <p>The iodine-tube was subsequently again tested, and it lighted-up better
    than on the last occasion, showing nearly the same effects in bulbs and
    capillary, the former having somewhat of a rosy tint and the latter an amber.</p>

    <div class="sidenote">Magnet
    effects. The spectrum
    described. Change
    when magnet
    was
    excited.</div>

    <p>On exciting the magnet, the capillary part of the tube changed from amber
    to a decided light green. The spectrum, without the magnet, gave one very
    bright line, and several less bright ones near, in the blue-green. The rest of
    the spectrum, with the exception of the absorption-spaces, was misty and continuous,
    with lines showing faintly through. The red and yellow portions of
    the spectrum were quite bright. When the magnet was excited, the
    spectrum entirely changed. The red and yellow portions of the spectrum,
    and the misty continuous light, all quite disappeared; while a set of
    sharp lines on the yellow-green and green flashed up bright and clear, and
    stood out alone upon a dark background, in which the absorption-spaces
    were lost. The effect was very strongly marked, and gave a totally different
    character to the appearance of the spectrum. The change seemed to arise
    from the suppression of one part of the spectrum, and the increase in intensity
    of the lines in the other part.</p>

    <div class="sidenote">No change
    in line-position.</div>

    <p>The principal lines could not be traced to change in actual position.</p>

    <p>This tube differing somewhat from a second one we examined (No. 2) in
    tint of glow and spectrum, it suggested itself to us that there might be a
    partial mixture of N or H (or both) with the iodine vapour, giving rise to
    some of the brighter parts of the spectrum which were extinguished under
    the action of the magnet.</p>

    <div class="sidenote">Comparison
    of iodine-tubes
    No. 1
    and No. 2. Comparison
    of the
    spectra.</div>

    <p>We therefore compared these two tubes, viz. the old one (No. 1) and the
    new one (No. 2), and also their spectra, by means of a comparison-prism on
    the slit of the spectroscope. To the eye, the tubes differed much in appearance.
    No. 1 had a distinct transparent rosy tint throughout, with considerable
    coarse flickering stratification; and this contrasted strongly with the
    dense whitish light of tube No. 2, which showed neither movement nor
    stratification. The spectra were also found different in general look. That
    of tube No. 1 was strongly tinged in the red and yellow, and showed a bright
    continuous spectrum, crossed by many sharp lines, with little trace of absorption-spaces.
    The spectrum of No. 2 was much whiter in tint, showed very
    little of the red and yellow, and the absorption-spaces were very dark. A
    few bright lines, mainly in the yellow-green and green, were faintly seen.</p>

    <div class="sidenote">The two
    tubes examined
    in
    detail.
    No. 2.</div>

    <p>The two tubes were then examined separately in detail. No. 2, excited by
    the magnet, showed curious effects. The glow was rendered weak and intermittent,
    and the rosy tint almost disappeared. The capillary changed to a
    decided green colour, and the positive electrode was surrounded by a yellow
    glow. The changes in the spectrum were no less decided. Without the
    magnet, the spectrum was found to be a bright continuous one of H (with a full
    set of principal and intermediate lines) and N—the N spectrum being rather
    faint and misty, with very slight, if any, traces of the iodine-spectrum. On
    the magnet being excited, the spectrum changed as if by magic; the H and N
    spectra disappeared (except hydrogen F, which still faintly remained), and the
    iodine lines, mostly in the yellow-green and green, shone out wonderfully
    sharp and bright on quite a dark ground. No. 1, upon examination, showed
    between the magnet-poles only the same changes as on last occasion. The
    spectrum seemed to be one of iodine, with the addition of slight traces of the
    H spectrum.</p>

    <div class="sidenote">Effects
    discussed.</div>

    <p>On excitation of the magnet, the misty continuous part of the spectrum
    nearly disappeared, and the bright lines shone up sharply upon the dark
    background as before. The effects in the case of both tubes were strongly
    marked. The impression as to tube No. 2 was that, without the magnet, the
    slight iodine-spectrum was overpowered and masked by the N and H spectra;
    while under the influence of the magnet the N and H spectra were almost
    altogether suppressed, the iodine-spectrum being at the same time intensified.
    The disappearance of the continuous spectrum under the action of the magnet
    in No. 1 (with the supposition it was mainly H) would be accounted for in the
    same way.</p>

    <h4 id="chap-14-9"><i>Bromine-tubes.</i></h4>

    <div class="sidenote">Bromine-tube
    No. 1. Lighting-up
    described. Effect of the
    magnet. Bromine-tube
    No. 2. Effect of
    magnet.</div>

    <p>This tube (No. 1) had been previously worked for photographic purposes.
    Excited by the small coil, the whole tube was filled with a faint flickering
    light. The positive bulb contained a faint purple glow, with a yellow-green
    tinge at the electrode, a curious flickering stream of light flashing from the
    electrode to the side of the tube. The negative pole showed pretty much
    the same effect as the positive. The capillary stream expanded at the opening
    into the positive bulb, but ran in a condensed stream into the negative
    bulb. In colour it was of a rather bright lilac. Upon putting the magnet
    on, the light-glow in the tube was at once and permanently extinguished,
    the coil still working as if the current passed. The same effect happened
    repeatedly; but now and then the tube lighted-up for a second, showing
    spiral arrangement in the bulb. We tried another bromine-tube (No. 2):
    it lighted-up easily; both bulbs were filled with a purple stream of light;
    capillary stream bright grey. The glass of the tube was strongly fluorescent
    and of a yellow tinge. When the magnet was excited the stream of light was
    somewhat condensed in the bulbs, and flew to the side of the tube; while
    the capillary stream at the same time brightened. The spectrum without
    the magnet was fairly bright; it increased in brightness under the influence
    of the magnet, and additional lines appeared; but we considered them to be
    only faint existing ones brightened up. No change in the position of the
    principal lines was traced.</p>

    <h4 id="chap-14-10"><i>Silicic-Fluoride tubes.</i></h4>

    <div class="sidenote">Si Fl₆ tube. Lighting-up
    described. Effect of
    magnet.</div>

    <p>(1) A tube marked Si Fl₆ had been worked for photographic purposes; it
    lighted-up easily. Both bulbs were filled with a brown-pink diffused light,
    inclined to condense into a stream in the positive bulb. The violet glow was
    very bright, and nearly filled the space round the electrode. The capillary
    stream was of a bright violet tint. The effect of the magnet was to decrease
    the intensity of the light throughout the whole tube.</p>

    <p>In the positive bulb the stream broke up into a number of vibrating
    streamlets, with little bright threads of light intermixed, which flew towards
    the side of the tube at right angles to the magnetic poles. There was an
    inclination to spiral arrangement in the streamlets. This stream changed from
    side to side of the tube coincidently with change in the magnetic poles. At the
    negative pole the violet glow formed an arc in the direction of the magnetic
    curves, while a spiral of fainter (positive?) light was formed in the upper
    part of the bulb. A slight ringing sound was heard in the tube.</p>

    <div class="sidenote">Comparison
    of Si F₄ and
    Si Fl₆ tubes.</div>

    <p>(2) We compared two tubes (Si F₄ and the one marked Si Fl₆). The Si Fl₆
    tube in general effect, and in its spectrum, when lighted-up, resembled Si F₄.
    We compared the one tube under the influence of the magnet with the other
    not so, by means of a comparison-prism on the slit. As the spectroscope and
    second tube were necessarily removed some distance from the magnet, the
    spectrum of the tube between the poles was not bright. We could not trace
    a change of position in any of the principal lines. The tube between the
    poles was brightened up when the magnet was in action&nbsp;<span class="footnote">The tubes generally seem marked Si Fl instead of the ordinary notation Si F. Si Fl₆ is probably, in fact, Si F₄.</span>.</p>

    <h4 id="chap-14-11"><i>Sulphuric-Acid (SO₃) tubes.</i></h4>

    <div class="sidenote">SO₃ tube
    No. 1;
    lighting-up
    described. Effect of the
    magnet. Changes in
    the spectrum.</div>

    <p>(1) Excited by the small coil, both bulbs of this tube (No. 1) lighted-up
    brightly, with a misty light-blue tinted stream of opaque light, a yellow glow
    appearing at the negative pole. The capillary stream partook of the same blue
    tint, but was whiter and brighter. Under the magnet’s influence, the glow
    in the bulbs flew to the side of the tube in flickering streams of light, the
    capillary at the same time changing to a distinctly green tint. The spectrum
    without the magnet consisted of four fairly bright bands of light in the yellow,
    green, blue, and violet, connected by a faint misty continuous spectrum (O or
    possibly the hydrocarbon spectrum found in O tubes by way of impurity).</p>

    <p>When the magnet was excited, this spectrum entirely disappeared; and a set
    of bright metallic-looking lines upon a dark background (line-spectrum of S)
    took its place. This effect was produced whenever the magnet was excited,
    and we tried it several times, to make sure of the complete change. After a
    time, when the magnet, battery, and the coil-power were all weaker, with the
    magnet on, we obtained a compound of both spectra, the bright lines being
    seen upon the continuous spectrum in which the bands appeared. When the
    magnet was taken off, the bright lines disappeared, and the O spectrum alone
    remained.</p>

    <div class="sidenote">SO₃ tube
    No. 2 examined.</div>

    <p>(2) We also tried another SO₃ tube (No. 2) which had been worked for
    photographic purposes, and was suspected of a carbon impurity. Without
    the magnet, the spectrum was very like that of the first tube; but when the
    magnet was excited, the spectrum only brightened, and no bright metallic-looking
    lines appeared.</p>

    <h4 id="chap-14-12"><i>Sulphur-tube.</i></h4>

    <div class="sidenote">Sulphur-tube. Lighting-up
    (without
    heating) described.</div>

    <p>(1) A small bent vacuum-tube containing some solid sulphur, excited by
    the smaller coil, and without being heated, gave a narrow stream of bright
    blue-green light running straightly through it. With the magnet on, this
    stream was deflected in the bulbs, and the capillary changed from a blue-green
    to a distinct rosy tint.</p>

    <div class="sidenote">Effect of
    magnet. Changes in
    the spectrum.</div>

    <p>Without the magnet, the spectrum consisted of four bright bands, with a
    continuous spectrum between, resembling that of SO₃ tube No. 1. With the
    magnet on, the spectrum brightened, especially in the yellow and red, which
    were dull before; and a set of lines appeared upon it (a line or band in the
    yellow especially showing) which were not seen before. The lines were distinct,
    but not very bright. The action on the capillary was noticed to be
    strongest just between the conical points of the armatures; and, in accordance
    with this, the central part of the spectrum-band in the red and yellow
    showed an increased brightness.</p>

    <div class="sidenote">Effects when
    one of the
    bulbs of the
    tube was
    heated. Changes in
    the spectrum
    under
    influence of
    magnet.</div>

    <p>(2) One of the bulbs of the tube was then gradually heated with a small
    gas-flame. The single stream in the heated bulb became somewhat deflected
    and broken up into a number of smaller streams; and these, when placed
    under the magnetic influence, had small spark-like threads of light running
    among them. The capillary, as the tube was heated, and the sulphur rose
    in it, changed somewhat in tint, and, under the magnetic influence, became of
    yellow-rose hue. As the heat was applied to the bulb the bands of sulphur
    gradually appeared in the field of the spectroscope, until at last the band-spectrum
    of sulphur entirely took the place of the spectrum seen in the cool
    tube. The magnet being excited, the spectrum changed at once, a set of
    bright sharp lines (line-spectrum of S) appearing upon a faint and dull image
    of the band-spectrum.</p>

    <p>This effect was constantly repeated upon the magnet being excited. The
    magnet being taken off, the band-spectrum alone was to be seen.</p>

  </section>

  <!-- Chapitre 15_________________________________________________________ -->
  <section class="chapter" id="chapter-15">
    <h3 class="titlechapter" id="chap-15">Effect of Magnet on a capillary Glass Tube</h3>
    <p class="shorter">Effect of Magnet on a capillary Glass Tube</p>

    <div class="sidenote">Capillary
    portion of a
    Geissler
    tube tested
    in three
    ways.</div>

    <p>The capillary portion of a Geissler tube was cut away from the bulbs,
    cleaned, and connected by a small vulcanite tube with the gas-pipe in the
    room conveying coal-gas at ordinary pressure. The flame was small and oval
    in shape, 8 millims. high, by 4 millims. wide, and burnt quite steadily. (Plate
    XVII. fig. 13.)</p>

    <div class="sidenote">No effect on
    flame.</div>

    <p>(1) The capillary tube was placed between the poles of the excited
    magnet, almost, but not quite, touching them; no effect at all was produced
    on the flame.</p>

    <p>(2) The tube was placed so that the conical ends of the armatures were
    allowed to compress the centre of it between them; still no effect was produced
    on the flame.</p>

    <p>(3) The tube was placed so that the straight sides of the armatures compressed
    it between them; still no effect took place on the flame.</p>

    <div class="sidenote">Flame between
    poles
    of magnet.</div>

    <p>(4) The flame itself was placed between the poles of the magnet. It was
    slightly drawn towards one pole with an inclination to form the magnetic
    curve.</p>

    <div class="sidenote">Quill glass
    tubing
    tested. No effect on
    the flame.</div>

    <p>(5) A piece of quill glass tubing was selected, 5 millims. in diameter and
    1 millim. thick, and drawn out to a point, the end of which was snapped off
    and the tubing connected as before. The flame was 20 millims. high, and
    5 millims. across, and somewhat lambent. On being placed (1) between the
    conical ends and (2) between the flat ends of the armatures, no effect could
    be seen on the flame.</p>

    <div class="sidenote">Effect on
    taper and
    spirit-lamp
    flames.</div>

    <p>(6) A small taper-flame was placed between the poles of the magnet: no
    effect was produced, except that the flame gave a slight “jump” each time
    the magnet was excited. A spirit-lamp flame was tried with a similar result.</p>

    <h4 id="chap-15-1"><i>Action of Magnet on a bar of heavy glass.</i></h4>

    <div class="sidenote">Heavy glass
    bar and
    mounting
    described.</div>

    <p>A piece of heavy yellow-tinted glass was selected, being a bar 10 centimetres
    in length, and 8 millimetres square. This was mounted in a frame
    with a Nicol prism at one end, and a double-image prism (next the eye) at
    the other.</p>

    <p></p>

    <div class="sidenote">Placed along
    poles of
    magnet. Effect of
    magnet on
    candle-images.</div>

    <p>(1) The glass bar and mounting were placed upon and along the poles of
    the magnet (in the direction of the magnetic curves), and the double-image
    prism and Nicol were so adjusted that two images of a candle were seen—the
    one below bright and normal, the one above, by rotation of the prism, as
    nearly as possible extinguished (Plate XVII. fig. 4). On exciting the magnet
    the faint image at once conspicuously brightened, at the same time assuming
    a slightly green tinge. To get full effect of brightening, it seemed necessary
    to have good pressure-contact between the battery-wires and the binding-screws.</p>

    <div class="sidenote">Effect on
    using a tourmaline
    as
    analyzer.</div>

    <p>(2) Using a tourmaline as analyzer in lieu of the double-image prism, the
    candle-flame was seen alternately brightened and darkened, as the tourmaline
    was rotated; and when the image was obscured by rotation, excitation of the
    magnet caused it to brighten strongly. This effect was accompanied by the
    apparent removal of a dusky red patch or spot, which occupied the centre of
    the field when the flame was obscured.</p>

    <div class="sidenote">Bar placed
    at right
    angles to the
    poles: no
    effect produced.</div>

    <p>(3) The bar of glass and double-image prism being placed between the
    conical ends of the armatures, but at right angles to, instead of along, the
    poles, upon excitation of the magnet no effect at all was produced.</p>

    <p>(4) The bar and prism being placed in the same position between the flat
    ends of the armatures, no effect at all was produced.</p>

    <div class="sidenote">Slight effect
    on second
    experiment.</div>

    <p>(4<i>a</i>) Experiment No. 4 was repeated. It was thought that on excitation
    of the magnet the secondary image slightly brightened; but there was a
    doubt about it, and the effect (if any) was slight.</p>

    <div class="sidenote">Effects produced
    when
    a biquartz
    was introduced.</div>

    <p>(5) The apparatus was now changed for one of the following arrangement:—1,
    a rotating Nicol prism next the eye; 2, the glass bar; 3, a biquartz
    with the halves horizontal; 4, another Nicol prism. The neutral-passage
    tint of the biquartz was found to be rather green (from mixture with the
    yellow of the glass).</p>

    <div class="sidenote">Change in
    colour of the
    halves.</div>

    <p>(i.) Placed <i>along</i> the poles of the magnet and the magnet excited, a change
    of tint was seen in both halves of the biquartz, the slightly purple-reddish
    tint of the upper half passing into a full purple. Effect not so marked as
    with the double-image prism.</p>

    <p>(ii.) Placed <i>across</i> flat ends of the armatures (as in experiment No. 4) no
    effect was seen.</p>

  </section>

  <!-- Chapitre 16_________________________________________________________ -->
  <section class="chapter" id="chapter-16">
    <h3 class="titlechapter" id="chap-16">Effect of Magnet on wide Air (Aurora) tube</h3>
    <p class="shorter">Effect of Magnet on wide Air (Aurora) tube</p>

    <div class="sidenote">Wide air-tube
    described.</div>

    <p>A large, wide air-tube was tried; it was 14½ inches long by 1 inch in diameter,
    of the same bore throughout, and with straight platinum electrodes.</p>

    <div class="sidenote">Magnet
    effect when
    tube placed
    vertically
    between
    conical armatures.</div>

    <p>(1) To excite it the larger coil was used. The tube was filled with bright,
    steady, rosy light, and beautiful stratification, which, as it flickered, seemed
    to incline to a continuous spiral (Plate X. fig. 8). This stratification was
    very close and fine, and extended nearly throughout the tube. On excitation
    of the magnet (the tube having been placed <i>vertically</i> between the conical
    armatures), the glow was condensed into a bright solid line or stream of light
    at the point which lay directly between the poles. This line or stream expanded
    into an elongated funnel-shape as it retreated from this centre towards
    the extremities of the tube, the stratification showing itself more distinctly
    as the glow of light became less dense (Plate XVIII. fig. 3). The stream of
    light was driven away at right angles to the poles, and changed from side to
    side of the tube with the direction of the current.</p>

    <p>[With the small coil this tube showed only a flickering stream of light,
    with very slight indications of stratification.]</p>

    <div class="sidenote">Effect when
    tube placed
    horizontally
    between the
    armatures.</div>

    <p>(2) The tube was placed <i>horizontally</i> between the conical ends of the
    armatures. The condensed stratified stream of light flew upwards and downwards
    (according to direction of current) instead of to the respective sides of
    the tube.</p>

    <div class="sidenote">Tube placed
    along the
    poles of the
    magnet.</div>

    <p>(3) The tube was placed along the poles of the magnet. In the interval
    between these the stream was driven upwards, but at either end sideways,
    right or left according to whether the pole was N. or S. (Plate XVIII.
    fig. 4). The result gave a complete spiral of stratified condensed light within
    the tube.</p>

    <h4 id="chap-16-1"><i>Note on Stratification.</i></h4>

    <div class="sidenote">Stratification
    in
    small tubes
    arranged in
    series.</div>

    <p>The current from the large coil was sent through a set of five small French
    vacuum-tubes, of equal calibre, containing salts of strontium and calcium,
    and showing phosphorescent effects. These tubes were arranged in single
    series; and, from the colour of the glow-discharge, were presumed to contain
    rarefied air in contact with the salts.</p>

    <p></p>

    <p>A strong coarse stratification was seen in the central (No. 3) tube. Tubes
    Nos. 2 and 4 also showed stratification, but in a less degree; while the outside
    tubes, Nos. 1 and 5, showed no stratification at all. The current was
    steady, and these effects did not fluctuate.</p>

    <h4 id="chap-16-2"><i>Effect of Magnet on Plücker (Air-) Tube.</i></h4>

    <div class="sidenote">Plücker
    air-tube. Lighting-up
    described. Effect of
    magnet on
    the positive-pole
    stream.</div>

    <p>(1) A Plücker air-tube was selected of the form shown on Plate V. fig. 1,
    and was excited by the small coil. The ring was used for the positive pole,
    the straight electrode for the negative. When lighted up, the tube glowed
    with a perfectly steady and quiescent light. The negative electrode was surrounded
    by the usual bright violet glow, extending itself and being gradually
    lost at a short distance from the wire, while the ring let fall a faint, tubular,
    salmon-coloured, diffused stream of light, which met the violet glow as it
    approached the negative pole. The tube was then placed vertically between
    the poles of the electro-magnet, the armatures being almost in contact with
    the sides of the tube around the negative pole. On excitation of the magnet,
    an instantaneous change took place. The stream of light from the positive
    pole contracted itself, so that it became of a long funnel-shape (the ring
    forming the mouth of the funnel), while it tapered almost to a point where
    it met the violet glow.</p>

    <div class="sidenote">Effect on
    negative
    violet glow.</div>

    <p>The stream also became very brilliant (the sides of the tube being left proportionately
    free from light), and crossing it were a set of bands, or striæ,
    having a waving or vibratory motion. The whole of the negative violet glow
    was simultaneously gathered into a brilliant narrow arc, which stretched
    across between the poles of the magnet. These effects are shown on Plate V.
    fig. 1. The edges of the arc were remarkably sharp and well defined, and
    with no surrounding aura or shading off.</p>

    <div class="sidenote">Arc of light
    followed the
    magnetic
    curves.</div>

    <p>By moving the tube between the armatures it was seen that the arc of
    light followed the magnetic curves. If the tube was moved upwards, the arc
    curved towards the zenith, if downwards, contrariwise; and a middle position
    could be selected, in which the edges of the arc were nearly parallel. Moving
    the tube a short distance from the pole had the effect of rendering the arc
    more diffuse, but not of otherwise altering its character.</p>

    <div class="sidenote">Direction of
    the current
    changed. Effects on
    glow described.</div>

    <p>(2) The direction of the current in the tube was then changed; and, without
    the magnet, the ring electrode was surrounded by a diffused violet glow;
    while the straight wire gave forth a faint salmon-coloured stream of light,
    spreading up to the ring.</p>


    <div class="sidenote">Magnet
    effects described. On negative
    pole. Rings from
    positive
    pole described. Effects on
    rings of
    making
    and breaking
    contact
    with
    magnet. Shape of
    rings described.</div>

    <p>On excitation by the magnet (the positive pole being now placed between
    the armatures), the violet glow of the negative pole contracted into a compact
    mass round the ring electrode. At the same time from the positive pole
    sprang a set of bright saddle-shaped rings, which increased in size as they
    advanced; and spreading upwards with a rapid but smooth motion towards
    the negative pole, closely approached to, but never actually came in contact
    with, the violet glow. The positive end of the tube was otherwise but
    slightly lighted, and the sudden appearance of this brilliant stream of rings
    of light was very striking. A single bright ray was also seen running from
    the positive wire, in a somewhat transverse course, along one side of the tube.
    When wire-contact with the magnet ceased, so that it was not excited, the
    rings ran back in succession to the positive pole and disappeared, and by
    making and breaking contact they were caused to advance and retire at will.
    They were accompanied by a waving or vibratory motion, and were evidently
    of the same character as the smaller striæ or bands mentioned as seen when
    the ring formed the positive pole. The general appearance was that of a
    hollow cone of light (the base towards the negative pole), composed of brilliant
    rings with dark spaces between, which appeared and expanded under
    the magnetic influence, and contracted and disappeared on its removal. The
    rings did not appear to be flat disks, but were somewhat curved or saddle-shaped.
    They reminded one much of the diatom <i>Campylodiscus spiralis</i>;
    that is to say, they were apparently flat if looked at from above, but like a
    figure of 8 when viewed sideways, the peak of the saddle forming a kind of
    brilliant point or apex.</p>

    <p>All this is difficult to describe; but an illustration from a sketch made of
    the tube is given on Plate XVII. fig. 2.</p>

    <div class="sidenote">Negative
    pole placed
    vertically
    on the
    magnet.</div>

    <p>(3) The negative pole (straight electrode) was then placed vertically on
    one of the poles of the electro-magnet. On excitation, the violet glow was
    contracted into a small upright brush or column of bright light, with a slight
    inclination to curvature.</p>

    <div class="sidenote">Tube laid
    horizontally
    across poles
    of magnet.</div>

    <p>(4) The same Plücker tube was laid horizontally across the poles of the
    electro-magnet (without armatures), the respective electrodes being above
    each pole.</p>

    <span class="sidenote">Effects produced.</span>

    <p>From the negative (straight electrode) pole sprang a dense and compact
    arc of violet light, in the direction of the magnetic curves, which terminated
    at the upper circumference of the tube, but which, if prolonged, would have
    followed the curves to the opposite pole. The stream from the positive pole
    was very considerably brightened, as in the other experiments, but did not
    appear in the form of rings or waves. It assumed that of a bright steady
    continuous glow, which formed round the tube a not perfectly continuous,
    but distinct and well-marked, spiral. This form of discharge seems connected
    with the peculiar contour of the rings mentioned in experiment 2. One
    might, indeed, conjecture the spiral-shaped glow to be a ring of light extended
    or drawn out towards the negative pole.</p>

    <div class="sidenote">Effects like
    those obtained
    by
    Gassiot.</div>

    <p>Experiment No. 2 seems in result very like that of Gassiot’s with his grand
    battery and the Royal Institution magnet, the effects (though of course upon
    a smaller scale) being similar to those obtained by him.</p>

    <h4 id="chap-16-3"><i>Effect of Magnet on Plücker Tube (Tin Chloride).</i></h4>

    <div class="sidenote">Plücker
    tube (tin
    chloride).
    Lighting-up
    described.</div>

    <p>A large Plücker tube was examined, which had a bulb attached at each
    end, communicating with the central portion by a narrow neck or constriction.
    On connexion with the small coil, a narrow stream of pale diffused
    cobalt-blue light ran along the whole tube, from point to point of the electrodes,
    the positive wire at the same time glowing with an aura of amber-yellow
    light. (See Plate XVII. fig. 3, where the narrow stream of light is
    shown by dotted lines.) At the two necks or constrictions the stream of
    light was perceptibly brightened.</p>

    <div class="sidenote">Effects of
    magnet
    upon the
    stream.</div>

    <p>When the magnet was connected, the stream in the positive bulb was not
    much changed, but only slightly bent. In the central partition of the tube
    and in the negative bulb, the stream of light was broken and split into a
    number of smaller streams, and at the same time bent or forced against the
    sides of the tube. (See Plate XVII. fig. 3.)</p>

    <div class="sidenote">Peculiar
    noise within
    the tube.</div>

    <p>In the central partition, the blue streamlets were accompanied by a number
    of spark-like threads of golden light, which shone out among them as the
    whole vibrated against the side of the tube; at the same time a peculiar
    pattering, as of a miniature hail-storm within the tube, made it ring with a
    slightly metallic tinkle.</p>

    <p>The direction of the bending or deflection of the stream was at right angles
    to the axis of the poles of the magnet, and changed from side to side of the
    tube as the direction of the current from the coil was varied.</p>

    <div class="sidenote">Spectrum
    described.
    Without
    magnet. With the
    magnet
    excited.</div>

    <p>In the positive bulb the stream, instead of joining the point of the electrode,
    left this and ran along one side of the whole length of the wire. (See effect,
    Plate XVII. fig. 3.) The spectroscope was applied to the neck of one of the
    bulbs where the stream was bright. Without the magnet a faint continuous
    spectrum, mainly of the blue and green, with very slight traces of the yellow
    and red, was seen. Upon this, five or six faint but sharp and metallic-looking
    lines were seen. On the magnet being excited, the continuous spectrum was
    not changed; but the sharp lines shone out brighter and clearer, one in the
    blue being especially conspicuous. These lines were measured with a micrometer;
    and their places being compared with Lecoq de Boisbaudran’s
    “Spectres lumineux,” they were easily recognized to be those of tin. On
    each excitation of the magnet the same brightening of the lines took place.</p>

    <h4 id="chap-16-4"><i>Effect of Magnet on Tin-Chloride Geissler Tube.</i></h4>

    <div class="sidenote">Geissler
    tube, Sn Cl₄,
    examined.
    Glow described. Effect of
    the magnet.</div>

    <p>We then examined a Geissler tube, marked Sn Cl₄. When first excited
    by the small coil, the spark passed freely. The glow in the bulbs was of a
    diffused, light purple tint; the positive electrode had a bright yellow glow
    around it. The capillary stream was of a sharp green-yellow, at times
    brightening up to a metallic-looking green. When the magnet was first
    employed, the tube distinctly and permanently brightened up throughout.</p>

    <div class="sidenote">Spiral
    formed in
    positive
    bulb. Glow in
    tube extinguished.</div>

    <p>The negative bulb was not much changed in appearance; but in the
    positive bulb a curious permanent and steady cloud-like spiral, of a purple
    colour, made its appearance, and lasted while the tube was under the magnetic
    influence. (See Plate XVII. fig. 12.) After a short time the tube seemed to
    lose a great deal of its conducting-power, and to light up in a feeble and
    intermittent manner, brightening only when the coil was made to work its
    best. While in this condition, the magnet (which had been previously disconnected)
    was excited, and at once what moderate glow was still shining in
    the tube was totally extinguished. At first it was thought some accident
    might have happened to the conducting-coil wires; but repeated trials satisfied
    us that the effect was due to the magnetic influence alone. Efforts were
    made, by looking to the coil and battery, to brighten up the tube as at first,
    but they quite failed; and it was evident some change had taken place in its
    conducting qualities. This tube was accidentally broken, so that we had no
    opportunity to renew the experiments.</p>

    <div class="sidenote">Another tin-chloride
    tube tried.
    Glow described.</div>

    <p>We subsequently tried another tin-chloride tube, purchased of Mr. Browning.
    This lighted up like the former tube, but brighter. There was an
    amber glow at the junction of the negative bulb which adjoined the capillary
    part. This was lost on putting on the magnet. At the same time a perceptible
    pattering ringing noise was heard in the tube, and metallic-looking
    threads of light ran through the bulbs.</p>

    <div class="sidenote">Spectrum
    described.</div>

    <p>Without the magnet, the spectrum was a continuous faint misty one, with
    bright lines of tin occasionally flashing up. With the magnet, the tin lines
    at once shone out bright, strong, and clear upon a black background, the
    change in effect being very marked.</p>

  </section>

  <!-- Chapitre 17_________________________________________________________ -->
  <section class="chapter" id="chapter-17">
    <h3 class="titlechapter" id="chap-17">Effect of Magnet on bulbed Phosphorescent Tube</h3>
    <p class="shorter">Effect of Magnet on bulbed Phosphorescent Tube</p>

    <div class="sidenote">Large phosphorescent
    bulbed tube. Lighting-up
    described. Spectrum
    described. Glow when
    discharge
    stopped, described.</div>

    <p>Mr. John Browning kindly lent me a large phosphorescent tube with five
    bulbs, said to be filled with anhydrous sulphurous-acid gas (SO₂). (See
    Plate XVIII. fig. 1.) This tube lighted up beautifully with the large coil.
    The connecting tubular parts of it were filled with a bright, beaded, transparent,
    rosy light; while the bulbs glowed with a more opaque blue-tinted effect. The
    spectrum of the tubular part was found to agree exactly with the principal
    bright band seen in a SO₃ Geissler tube. The spectrum of the bulb-glow
    was a faint green-blue continuous one, with bright bands or lines faintly flashing
    up at times. When the discharge was stopped, the tube still glowed with
    a moderately bright, opaque, grey-green light. This glow gradually faded out,
    always commencing with the bulb forming the negative or violet pole, and so
    dying out, bulb by bulb, towards the positive pole. The negative-pole bulb
    at times was, on suddenly stopping the current, hardly lighted at all, the other
    bulbs being luminous.</p>

    <div class="sidenote">Comparison
    with SO₃
    Geissler
    tube.</div>

    <p>(1) We compared the large tube with a SO₃ Geissler tube, by means of a
    comparison-prism on the slit, with the result before detailed. The Geissler
    tube, however, showed no after-glow.</p>

    <div class="sidenote">Effects in
    bulbs on
    lighting-up
    the tube
    described. Effects of
    reversal of
    the current. After-glow
    restored by
    passing of
    current.</div>

    <p>(2) We lighted up the Browning tube with the large coil. The negative
    bulb was always the least filled with the blue opaque vapour, and the other
    bulbs increased in vapour-density in the order they approached towards the
    positive bulb. When the current was reversed, so that the negative and positive
    glow changed places, the negative bulb still remained transparent,
    although the positive opaque glow had (presumably) been thrown into it.
    When the after-glow had quite disappeared in the bulbs, it was again
    strongly restored, by the passing of the current for a few seconds only through
    the tube.</p>

    <div class="sidenote">Effect of
    reversal of
    current on
    positive-pole
    glow.</div>

    <p>(3) The tube was well excited, and the four bulbs (other than the negative
    one), upon stopping the current, glowed strongly. The current was then sent
    through reversed, so as to throw the negative glow for a few seconds into the
    positive bulb. The after-glow in the positive bulb was at once extinguished.
    On once more reversing the current, it was only restored after a certain
    amount of continuance of the positive stream.</p>

    <figure class="plate" id="plate18">
      <!-- <img src="assets/aurorae/images/plate18.jpg" /> -->
      <figcaption></figcaption>
    </figure>

    <p></p>

    <div class="sidenote">Effect of
    change of
    current on
    the three
    central
    bulbs.</div>

    <p>The time during which the negative glow was thrown into the positive bulb
    did not appear sufficient to have heated it. After rapidly changing the
    direction of the current several times and then stopping it, the three central
    bulbs alone had an after-glow, the two extreme ones having none, being both
    equally transparent.</p>

    <div class="sidenote">Effect of
    heat on the
    bulbs. Effect of
    cooling by
    ether-spray.</div>

    <p>(4) A moderate heat from a spirit-lamp was applied to the centre bulb (<i>a</i>)
    while the current was on; and also (<i>b</i>) when this was stopped, and the bulb
    glowed. In the first case the bulb was found to get more transparent; and in
    the second case the after-glow disappeared in a proportionately shorter time
    in the heated bulb than in the others. To test the result of cooling the bulbs,
    the negative-pole bulb and also the central one were each subjected to the
    action of ether-spray, and also of ether and water-spray mixed. This was
    done, (<i>a</i>) when the current was passing, and (<i>b</i>) when it was stopped and the
    glow only was in the bulb. The bulbs were cooled until a marked cold effect
    to the touch was produced. We did not notice any difference in the behaviour
    of the bulbs so treated as compared with the others, either when the current
    was passing or in the case of the after-glow.</p>

    <div class="sidenote">Negative-pole
    bulb
    between the
    armatures
    of magnet. Effects on
    negative
    and positive
    glow.</div>

    <p>(5) We placed the negative-pole bulb between the conical points of the
    armatures, and excited the magnet. The negative glow contracted itself into
    a condensed violet-tinted crescent, in accord with the magnetic curves. The
    positive glow of the same bulb lost its beaded (stratified) character, and was
    condensed into a bright stream of light, which latter protruded from the small
    inner tube and formed a spreading spiral set of cloud-rings within the bulb
    (see Plate XVIII. fig. 2). The action of the magnet seemed to be exercised
    in subduing the stratification, condensing the glow into a bright stream of
    light, and forcing the latter to “tail over” at each extremity of the tubular
    joints into the bulbs—this effect extending even so far as the second bulb.</p>

    <p>When the positive bulb was placed between the poles of the magnet, the
    glow was simply condensed into a bright stratified stream, which flew to either
    side of the bulb.</p>

    <div class="sidenote">Effect of
    magnet on
    glow in bulb
    No. 4.</div>

    <p>(6) <i>a.</i> Bulb No. 4 (see Plate XVIII. fig. 1) was placed between the poles
    of the excited magnet, and the current was passed and then stopped. The
    glow in that bulb faded away out of its order, and earlier than in ordinary
    cases (nearly as soon as No. 2).</p>

    <div class="sidenote">Other bulbs
    tested in
    similar
    manner.</div>

    <p><i>b.</i> The same and other bulbs were tested in a similar manner. In all cases
    the bulb influenced by the magnet, when the current was stopped, was found
    perceptibly fainter in after-glow.</p>

    <div class="sidenote">Effect of
    magnet
    upon the
    after-glow
    itself.</div>

    <p><i>c.</i> The tube was arranged with one of the bulbs between the poles of the
    unexcited magnet; the current was passed and stopped, and the after-glow
    obtained. The magnet being then quickly excited, the after-glow in the bulb,
    under its influence, faded out; and the bulb became transparent, perceptibly
    sooner than under ordinary circumstances. We tried this several times, with
    the same result in each case.</p>

    <div class="sidenote">Mr. Thompson’s
    experiments
    on
    action of
    magnets
    upon liquid
    rings.</div>

    <p><i>Note.</i>—In relation to these experiments, it may be mentioned that Mr. S.
    P. Thompson, of Bristol, is reported to have studied the action of magnetism
    upon rings of coloured liquid projected through water, and to have observed
    their retardation and partial destruction in passing through a powerful magnetic
    field.</p>

    <div class="sidenote">Mr. Ladd’s
    explanation
    of some of
    the phenomena
    observed.</div>

    <p>Mr. Ladd has suggested to me that some of the phenomena produced indicate
    a driving of the gas in the direction from the negative to the positive
    pole—a theory which is supported by the action of the magnet on the bulbs,
    if this be considered a repulsive one as regards the gas influenced.</p>

    <h4 id="chap-17-1"><i>Effect of Magnet on small Phosphorescent (powder) Tubes.</i></h4>

    <div class="sidenote">Tubes containing
    phosphorescent
    powders
    described.</div>

    <p>We examined six vacuum-tubes containing phosphorescent powders, which,
    upon exposure to sunlight and removal to the dark, or after passing of the
    electric current over them, continued to glow in the tubes after the exciting
    cause had ceased. They were of thin glass, and of equal calibre
    throughout.</p>

    <p>One was 6½ inches long and ⅝ inch in diameter, and had no label;
    the other five were 7½ inches long and ½ inch in diameter, and were labelled
    respectively:—</p>

    <ul>
    <li>Strontium vert,</li>
    <li><span class="ditto2">”</span> jaune,</li>
    <li>Calcium violet,</li>
    <li><span class="ditto2">”</span> orange,</li>
    <li><span class="ditto2">”</span> vert-bleuâtre.</li>
    </ul>

    <div class="sidenote">Lighting-up
    of the tubes
    described. Effect of
    magnet on
    ⅝-diameter
    tube. Spectrum
    without
    magnet.</div>

    <p>The powders in tubes of this description are said to contain either sulphide
    of strontium, or calcium, or sulphate of quinine. The first-mentioned tube
    shone with a white and bright light, and probably contained the latter substance.
    The general effect of the current on the tubes was similar in all
    cases. Under a sufficiently strong current, they lighted up with a brilliant,
    slightly green-white glow; in which, however, by looking sideways, it was
    possible to detect a delicate rosy tint. Any colours beyond these in the tubes
    seemed to depend on the powders enclosed in them. When the current was
    stopped, the powders alone glowed in accordance with the colours mentioned
    on the labels, the rarefied gas or air in the tubes not giving any after-glow, as
    in the case of the sulphurous-acid tube. When the ⅝-diameter tube was
    excited by the small coil, the effect of the magnet was to entirely suppress
    and extinguish the glow. When this and the other tubes were worked with
    the larger coil, the spectrum, without the magnet, was bright and continuous,
    either showing no lines or else very faint traces of them, and, extending through
    the whole range of colours was brightest in and about the green.</p>

    <div class="sidenote">Magnet
    effect on
    glow. Same on
    spectrum.</div>

    <p>With the magnet excited, a bright line of pink light was condensed against
    the upper side of the tube; while the glow in the tube generally became very
    decidedly fainter, except at the electrodes, which still preserved a certain
    amount of brilliancy. The spectrum also was much changed. The bright
    continuous glow became much fainter, and many sharp and fairly bright lines
    were seen upon it. These lines were, as to character, not easy to recognize.
    Hydrogen (F) was, however, plainly distinguished; and other lines, which we
    considered to be N, were common to all the tubes. Some lines were also
    remarked as being, without the magnet, not so constant.</p>

    <div class="sidenote">Tubes examined
    and
    compared
    for spectra.</div>

    <p>Calcium orange and calcium violet, compared for spectra, were identical;
    the two strontium tubes hardly so, but with strontium vert a bright continuous
    spectrum mainly hid the lines.</p>

    <p>Strontium jaune and calcium orange were not alike; strontium vert and
    calcium violet differed. Calcium orange and calcium vert-bleuâtre were
    considered alike; but the comparison was not easy, as the calcium vert
    was bright, and the lines were only seen faintly upon the continuous
    spectrum.</p>

    <p>In order not to shift the powders, the tubes were laid horizontally, and two
    spectra simultaneously examined across the tubes.</p>

    <h4 id="chap-17-2"><i>Lighting-up Tubes with One Wire only (Marquis of Salisbury’s Observations).</i></h4>

    <div class="sidenote">One wire
    only connected
    with
    an electrode.</div>

    <p>The vacuum-tubes employed were examined in the usual way, but one wire
    only was connected with an electrode. The other wire was attached to the
    end of a glass rod, and circuit was from time to time completed while the
    tube was before the spectroscope.</p>

    <p>The large coil was used. In all cases, with the one wire, the glow was very
    faint as compared with that of the closed circuit.</p>

    <span class="sidenote">Ether vapour.</span>

    <p>(1) <i>Ether Vapour.</i>—With both wires, in company with the usual bright
    bands of the carbon spectrum, shading-off towards the violet, the H lines were
    very sharp and brilliant. With the one wire only, the carbon bands were left
    faintly shining, with both sides nebulous alike, and with no shading-off towards
    the violet. (We were not quite sure whether this was not the effect of the
    reduction of the light.) The H lines, though originally stronger than the
    carbon bands, quite disappeared from the spectrum.</p>

    <span class="sidenote">Coal-gas.</span>

    <p>(2) <i>Coal-gas.</i>—The same effects were produced; but we thought we could
    detect very faint traces of the H lines.</p>

    <span class="sidenote">Nitrogen.</span>

    <p>(3) <i>Nitrogen.</i>—The N lines, as well as those of H (also seen in the
    tube), were much fainter with one wire, but the H lines more so in proportion.</p>

    <span class="sidenote">Hydrogen.</span>

    <p>(4) <i>Hydrogen.</i>—Only a marked reduction in brilliancy of the whole
    spectrum.</p>

    <div class="sidenote">Oxygen, N
    and H.</div>

    <p>(5) <i>Oxygen.</i>—An impure tube, showing O (some of the lines hydrocarbon?),
    N, and H spectra simultaneously. With one wire the O lines still remained
    fairly bright, the N and H being only faintly seen.</p>

    <span class="sidenote">Water-gas.</span>

    <p>(6) <i>Water-gas.</i>—Same effect.</p>

    <div class="sidenote">Turpentine
    vapour.</div>

    <p>(7) <i>Turpentine Vapour.</i>—Same effect as ether, but the H lines could be
    faintly seen.</p>

  </section>

  <!-- Chapitre 18_________________________________________________________ -->
  <section class="chapter" id="chapter-18">
    <h3 class="titlechapter" id="chap-18">Action of the Magnet on the Electric Spark</h3>
    <p class="shorter">Action of the Magnet on the Electric Spark</p>

    <div class="sidenote">Apparatus
    employed.</div>

    <p>The magnet was excited with two plates of the large battery, and the larger
    coil with the other two plates, the action in both cases being strong.</p>

    <p>1. A spark from the coil was passed between two platinum wire electrodes,
    about three centimetres apart.</p>

    <div class="sidenote">Spark
    and aura
    described.</div>

    <p>It consisted centrally of a thin stream of bluish-white light, vividly bright,
    around which was seen a narrow, uniform, diffuse, yellow-tinted aura, which
    accompanied the spark in all its movements. The spark always struck
    across from the extreme points of the electrodes (see Plate XVII. fig. 5).</p>

    <div class="sidenote">Effect of
    magnet
    upon the
    aura.</div>

    <p>2. On being placed between the conical poles of the excited magnet the
    bright thread of the spark did not change; but instead of the inconsiderable
    yellow-tinted aura which accompanied the unmagnetized spark, there now
    struck out, at right angles to the magnet-poles, a thin rosy-tinted half-disk
    of aura-like flame. This extended aura ran considerably along each electrode,
    though the spark proper still struck from the points.</p>

    <div class="sidenote">Extended
    aura described.</div>

    <p>The aura was somewhat larger in extent upon one electrode than on the
    other. In the first case, it sprang from a considerable number of minute
    illuminated points; on the other electrode, these illuminated points were
    fewer in number, and the flame was more purple in tint. Reversing the
    current these effects were reversed. The aura was uniformly thin and disk-like,
    and the curved edge remarkably true in shape (see Plate XVII. fig. 6).</p>

    <p>The lateral direction of the aura was changed when the current was
    reversed.</p>

    <div class="sidenote">Aura not
    proportionate
    to
    length of
    spark.</div>

    <p>3. The aura was found not proportionate to the length of the spark.
    When the electrodes were approached, so as to very much shorten the spark,
    the aura still sprang out to a distance and extent quite out of proportion to
    the length of the spark. Even when the electrodes were approached so
    close that the spark was very short indeed, still, under the magnetic influence,
    a very considerable aura made its appearance.</p>

    <div class="sidenote">Effect of
    working coil-break
    upon
    the aura.</div>

    <p>4. Upon working the coil-break, it was found that in proportion as the
    contact screw was drawn apart from the break, so the aura gradually
    diminished in extent, until at last, by continuing to increase the distance
    between the screw and the break, a point was reached when thin bright
    sparks, without any aura, passed. Upon the screw being worked up closer,
    thicker sparks passed, and the aura again made its appearance. As the
    aura diminished in size it gradually changed in tint from yellowish rose-pink
    to purple.</p>

    <div class="sidenote">Spark taken
    in glass
    bulb.</div>

    <p>5. The spark was taken in a glass bulb, the tube in which it was blown
    being open at both ends, with the same effect as in the open air.</p>

    <p>6. A plate of glass was laid on the poles of the magnet, and the spark was
    passed <i>along</i> the poles (in the same direction as the heavy glass was laid in
    the Faraday experiment). No aura was formed. The points were then
    moved round, so as to carry the spark at right angles to the poles, and the
    aura was formed as before.</p>

    <div class="sidenote">Aura could
    be blown
    away from
    the spark.</div>

    <p>7. The aura, it was found, could be blown away at right angles to the
    spark. When strongly urged, it assumed the shape of a flickering tongued
    curtain of flame, flying away in the contrary direction to that from which the
    current of air proceeded, and again returning to its original shape as the
    impulse was removed. The spark proper was not influenced (see Plate XVII.
    fig. 8).</p>

    <div class="sidenote">Effect of
    withdrawing
    spark from
    central position
    between
    the poles.</div>

    <p>8. As the spark was withdrawn from its central position between the
    poles of the magnet, the convex edge of the aura became gradually less
    perfect, and assumed a ragged and broken-up appearance, the inequality at
    times amounting almost to jets or flickering sprays of light. The spark was
    also slightly curved away from the electrodes (see Plate XVII. fig. 7).</p>

    <div class="sidenote">Magnet had
    no effect
    upon condensed
    spark.</div>

    <p>9. A condenser of four coated plates was introduced into the circuit, causing
    a sharp brilliant blue-white spark, apparently divided into streams and with
    no aura. The magnet had no effect whatever upon this form of spark.</p>

  </section>

  <!-- Chapitre 19_________________________________________________________ -->
  <section class="chapter" id="chapter-19">
    <h3 class="titlechapter" id="chap-19">The Discharge <i>in vacuo</i> in Larger Vessels, and Magnetic Effects thereon</h3>
    <p class="shorter">The Discharge <i>in vacuo</i> in Larger Vessels</p>

    <p>A Tate’s air-pump was used, and the spark from the larger coil. The
    exhaustion could not be carried very far.</p>

    <div class="sidenote">Globular
    receiver
    described. Discharge
    described.</div>

    <p>(1) A globular receiver was used, having brass caps for exhaustion, and
    platinum wires passing through the opposite sides for electrodes (see Plate
    XVIII. fig. 6). With partial exhaustion, from the positive electrode proceeded
    long, sharp, bright, rosy sparks, striking in zigzags across the receiver.
    From the negative terminal sprang a larger number of bluer and more diffuse
    streams of light, like spiders’ webs; and these were enveloped, for a short
    distance from the terminal, in a slight misty aura. Both sets played round
    the sides of the glass as well as across.</p>

    <div class="sidenote">Bell-shaped
    receiver
    described. Discharge
    described.</div>

    <p>(2) A bell-shaped receiver, with terminals inserted at the sides and one
    also at the top, was next used (see Plate XVIII. fig. 9). When the side
    terminals were employed, the effect was much the same as in the last case.
    When the top terminal was used for one wire (the other wire being connected
    with the pump-plate) a single stream of bright rosy light ran from the upper
    terminal to the plate. First striking the central part of the plate, the stream
    then glided towards one of the lateral terminals, and so to the edge of the
    receiver. After partly discharging itself by contact with the terminal, the
    stream as rapidly retreated to the centre of the plate again—this effect being
    from time to time repeated while the current was passing. The current
    being reversed, a number of bright, but weaker and more diffused, streams of
    light had the appearance of shooting from the upper electrode, and of striking
    upon the plate below; with a tendency to fly off from where they struck, in
    a similar manner to the single stream before described. Where each stream
    touched the plate a brilliant point of light appeared, and a strong pattering
    noise was heard in the receiver.</p>

    <div class="sidenote">Bell-shaped
    receiver
    without
    electrodes. Induction
    discharge
    described.</div>

    <p>(3) Another bell-shaped receiver of similar shape was used. This had no
    electrodes forming a direct communication with the interior; but, in lieu of
    these, two wafers of thin sheet brass were cemented, one inside and one outside
    the glass, opposite to one another. On connexion being made with the
    outside wafer, the effects produced by induction were similar to, and very
    nearly as strong as, those in the cases where direct communication with the
    interior of the receiver was made.</p>

    <div class="sidenote">Long large
    tube exhausted
    and
    illuminated. Spiral form
    of discharge.</div>

    <p>(4) A large tube, 24 inches long and 2 inches in diameter, with ball and
    point electrodes respectively, was exhausted, and the current passed through
    it. The effects were similar in most respects to those produced in the
    globular and bell receivers, but the streams of light assumed a distinctly
    spiral form in their passage (see Plate XVIII. fig. 5). This tube when placed
    between the poles of the magnet showed no effect, except a slight condensation
    of the streams of light towards the sides of the glass.</p>

    <div class="sidenote">Globular
    receiver
    again used.</div>

    <p>(5) The globular receiver first described was again used (the Tate pump
    having been cleaned and working easier).</p>

    <div class="sidenote">Phosphorescent
    after-glow
    succeeding
    the
    spark.</div>

    <p>(<i>a</i>) When exhaustion was as good as it could be got, the spark struck
    across in a single, slightly expanded, stream of rosy light, having a tendency
    to curve upwards (see Plate XVIII. fig. 6). The electrodes had but little
    glow round them, only just enough to distinguish the poles apart. When
    the flow of the stream was interrupted by breaking contact with one terminal,
    so that sparks passed in succession, we thought we detected a faint blue
    phosphorescent after-glow succeeding each spark.</p>

    <div class="sidenote">Positive
    wire only
    attached.</div>

    <p>(<i>b</i>) The positive wire only was attached to one electrode, the negative
    being unconnected. A set of faint whity-blue cobweb-looking streams of
    light spread from the electrode all over the receiver, having a vibratory
    motion. The spaces between these were dark, and there was no aura—the
    effect being similar, but not quite so bright and pronounced, as when both
    wires were attached (see Plate XVIII. fig. 7).</p>

    <div class="sidenote">Negative
    wire only
    attached.</div>

    <p>(<i>c</i>) The negative wire only was attached. The cobweb streams were
    absent, or only shot out very occasionally. The main effect was a straight
    nebulous stream of violet light, which commenced at the electrode and spread
    out in a fan-shape towards the lower brass cap of the receiver; while, at the
    same time, an aura or glow of similar light, but fainter in quality, spread
    from the electrode over at least one half of the receiver. This aura would
    no doubt have filled a small flask (see Plate XVIII. fig. 8).</p>

    <div class="sidenote">Effect of
    gradual
    exhaustion
    on the
    discharge.</div>

    <p>(<i>d</i>) When exhaustion was first commenced, both electrodes of the receiver
    (both wires being connected) threw out spider-web-like streams, as in Experiment
    1, pale blue from the one pole and somewhat rosy from the other.</p>

    <p>As the exhaustion progressed the pale-blue streams disappeared, while the
    rosy flickering ones diminished in quantity and extent until ultimately a
    single rosy stream of light crossed the receiver as in Experiment 5<i>a</i>. Upon
    admitting the air, these effects took place in an inverse order—the single
    stream being gradually broken up, and the spider-webs taking its place.</p>

    <div class="sidenote">Globular
    receiver
    placed on
    poles of the
    magnet. Magnet
    effect.</div>

    <p>(<i>e</i>) The exhausted globular receiver was placed upon the poles of the
    excited magnet, with the stream at right angles to them. Looking across the
    S. pole of the magnet, the negative electrode was on the left hand, and the
    positive on the right. The effect of the magnet on the stream was apparently
    to split it up into several; but this appearance must have been due to vibration
    only, as a revolving mirror showed the stream as single. When the
    current was reversed, the stream which, without the magnet, was somewhat
    flickering and vibrating, slightly straightened at the positive pole, and the
    whole stream became steadier.</p>

    <div class="sidenote">Single wires
    attached.</div>

    <p>(<i>f</i>) Single wires were successively attached to the negative and positive
    poles, and the cobweb streamers and glow before described obtained. The
    magnet was found to have no decided effect on either of these.</p>

    <div class="sidenote">Plücker tube
    placed between
    poles
    of magnet
    with negative
    wire
    only attached.</div>

    <p>(6) The Plücker air-tube (Plate XVII. fig. 2) was placed between the
    poles of the magnet, and the negative wire only was connected with the
    straight electrode. A pale violet glow was seen round this electrode, and
    another, but rather fainter, glow of a similar description at the ring electrode,
    the intermediate space being filled with a salmon-coloured light. This violet
    glow was condensed into an arc by the action of the magnet. Reversing the
    current, the violet glow still remained at each electrode, and that between
    the poles of the magnet was still influenced into an arc.</p>

    <div class="sidenote">Geissler
    tube substituted,
    with
    similar
    results.</div>

    <p>(7) An air Geissler tube was substituted for the Plücker tube, with very
    much the same result. Whichever wire was attached, a violet glow appeared
    at the connected electrode, and a fainter one of the same character at the
    other; and the magnet influenced both. The connecting salmon-coloured
    glow was faint.</p>

    <div class="sidenote">Globular
    receiver
    treated with
    phosphoric
    anhydride.</div>

    <p>(8<i>a</i>) The globular receiver had some phosphoric anhydride shaken into
    it; and it was then exhausted. The cobweb streamers and violet glow each
    appeared according to which wire was connected. There was no marked
    difference between the receiver with the anhydride and without; except that in
    the former case the streamers and glow were reduced in extent and strength,
    and were comparatively faint.</p>

    <div class="sidenote">Discharge
    in water-vapour
    described.</div>

    <p>(<i>b</i>) The anhydride having been washed out, first with plain and afterwards
    with distilled water, some drops of the latter were allowed to remain in the
    receiver. On exhaustion a vapour-cloud was formed, and the discharge passed
    (both terminals being connected with the coil) through this. The rosy stream
    of light was formed as usual, but was more flickering and unsteady. As the
    exhaustion was lessened, the rosy stream disappeared, and the cobweb streams
    began to fill the receiver. These were, however, not so bright and sharp as
    in a dry receiver, but were faint and broad; while some diffused and nebulous
    streams of light, running (slightly bent) from pole to pole, and from ¼ to ⅜ of
    an inch broad, were intermixed with them. When one wire only was connected,
    the glow and streamers from the electrode were very faint.</p>

    <div class="sidenote">Large bell-receiver
    and
    plate
    described. Receiver
    exhausted
    and stream
    of light
    formed.</div>

    <p>(9) A large bell-shaped receiver, 11 × 8 inches, was next used. It was
    open at the bottom, which was ground as usual; and had a small opening at
    the top, also carrying a ground edge. A solid brass plate was prepared,
    ground only round the edge (in order to take the receiver), and in the centre
    of this brass plate were inserted two disks of soft iron, corresponding in
    position and size with the poles of the Ladd electro-magnet (see Plate XVIII.
    fig. 11). When this plate was placed on the magnet, the poles and disks were
    in contact; and the disks became N. and S. poles within the receiver. A
    small brass plate carrying a tap and exhaust-tube, and a binding-screw for
    attaching an electrode within, closed the receiver at top. The receiver and
    plate being placed on the magnet-poles, the former was exhausted until the
    discharge became a rosy slightly-diffused stream of light; with a small unilluminated
    space between it and the negative pole, where the usual violet
    glow appeared round the wire.</p>

    <p>This stream of light was used for the experiments after detailed. In some
    cases the conical armatures were placed within the receiver, in others the
    disks alone were used as the magnetic poles.</p>

    <div class="sidenote">Effect on
    same when
    magnet
    excited.</div>

    <p>(<i>a</i>) With the apparatus arranged as shown on Plate XVIII. fig. 10, and
    the magnet excited, a violet glow appeared round the end of the wire
    which was negative. A small unilluminated space then intervening, the
    stream ran in a curve between the wire and the armature, which latter was
    positive. The stream was not steady and had a tendency to rotate; but as
    this was better observed with the disks only, it is described further on.</p>

    <div class="sidenote">Experiments
    with
    the conical
    armature
    removed. Vibrating
    stream.</div>

    <p>(<i>b</i>) The conical armature within the receiver was removed, and the stream
    allowed to connect with the centre of the pump-plate. When the magnet
    was excited, the stream was violently projected at right angles to the poles,
    with a vibrating movement to either side according to the direction of the
    current. When the wire was positive the movement was towards the left,
    with a slight inclination towards the N. pole. When the wire was negative
    the movement was to the right, but in a rather strong curve towards the
    N. pole. The vibrating motion was very distinct, and gave the appearance
    of six or seven streams running off at regular intervals (see Plate XVIII.
    fig. 12).</p>

    <div class="sidenote">Rotating
    wire over
    S. pole.</div>

    <p>(<i>c</i>) The wire was next placed over the centre of the disk forming the S.
    pole. With the wire negative and the pole positive, rotation of the stream
    was decidedly, but not very strongly, from right to left from the centre of the
    plate (as the hands of a watch). With the wire positive and the pole
    negative, rotation was strongly left to right, with a disposition to spiral twist
    in the stream (see Plate XVIII. fig. 13).</p>

    <div class="sidenote">Same over
    N. pole.</div>

    <p>(<i>d</i>) The wire was placed over the disk forming the N. pole. With the
    wire negative and the pole positive, rotation of the stream was left to right.
    With the wire positive and the pole negative, rotation was right to left (see
    Plate XVIII. fig. 14).</p>

    <div class="sidenote">Stream
    thrown
    across the
    receiver
    above the
    magnet-poles.</div>

    <p>(<i>e</i>) The stream was thrown across the receiver from the lateral binding-screws
    above, and at right angles to, the disks, and afterwards in the opposite
    direction, <i>i. e.</i> along them. In neither case was there any marked change
    when the magnet was excited.</p>

    <p>(<i>f</i>) The conical armatures were placed with the pointed ends upon the
    disks in the receiver, and the stream thrown above and along them. It
    diverged—one part running straight across between the electrodes, whilst
    another stream and some cobwebs ran from each electrode to its nearest pole.
    The streams and cobwebs flickered a good deal. There was no marked
    change when the magnet was excited.</p>

    <h4 id="chap-19-1"><i>Some of Baron Reichenbach’s Magnetic Researches tested.</i></h4>

    <div class="sidenote">Baron Reichenbach’s
    researches.</div>

    <p>In 1846 Dr. W. Gregory published an abstract of Baron Reichenbach’s
    ‘Researches on Magnetism and on certain allied subjects, including a supposed
    new Imponderable.’</p>

    <div class="sidenote">Auroræ considered
    to be
    magnetic
    lights. Flames
    seen by
    “sensitive”
    persons.</div>

    <p>From a paragraph in this work, it would seem that the Baron considered
    his observations as tending to an explanation of the Aurora Borealis; and,
    since it was generally admitted that these phenomena occur within our
    atmosphere, that there appeared a great probability of Auroræ being visible
    magnetic lights. The Baron, in the original work, fully describes the Aurora
    Borealis; and concludes it must be similar in its nature to the flames of light
    seen streaming from the magnet-poles by Mdlle. Reichel and other sensitive
    patients of the Baron’s. It is unfortunate that these flames were only seen
    by certain “sensitive” persons. The drawings given of them, too, show no
    analogy to the magnetic curves.</p>

    <p></p>

    <div class="sidenote">Magnet
    tested for
    such flames.</div>

    <p>Having the opportunity of a powerful magnet in that used during our
    tube-experiments, we made an attempt to detect the Baron’s magnetic flames,
    on or around the poles of our magnet, in a perfectly dark room. Arrangements
    were made to silently connect and disconnect the battery with the
    magnet, without the knowledge of any one except the operator. The experiment
    proved a complete failure; no flames or discharges of light of any kind
    were to be seen. The observers were five in number, two gentlemen and
    three ladies, but not one of the party proved “sensitive.”</p>

    <div class="sidenote">Mr. Brooks’s
    experiments
    on action of
    the magnet
    on a sensitive
    photographic
    plate.</div>

    <p>Some experiments made by Mr. W. Brooks, and detailed in a paper read
    by him before the South London Photographic Society, seem to corroborate
    (to a certain extent) the statements made by the Baron in regard to the
    influence of the magnet on a sensitive photographic plate.</p>

    <p>Remembering, however, how it has been demonstrated that light may be
    “bottled up” as an actinic source for a considerable period of time, it seems
    a question whether the images obtained were not due to some such source
    rather than to any magnetic aura.</p>

    <p></p>

    <h4 id="chap-19-2">SUMMARY OF THE FOREGOING EXPERIMENTS AND THEIR RESULTS.</h4>

    <div class="sidenote">Summary of
    the experiments.</div>

    <p>Chapter XIV. Action of magnet on glow and spectrum of Geissler gas
    vacuum-tubes demonstrated.</p>

    <p>Chapter XV. Action of magnet on glass capillary tube negatived.
    Faraday’s experiment with heavy-glass bar repeated.</p>

    <p>Chapter XVI. Action of magnet on glow in wide air-tube demonstrated.
    Note on stratification. In Plücker tube, action of magnet on negative pole
    (arc formed) and positive pole (Gassiot’s rings produced) demonstrated.
    Effects of magnet upon glow and spectrum of tin-chloride vacuum-tubes
    demonstrated.</p>

    <p>Chapter XVII. Effect of magnet upon after-glow in a bulbed phosphorescent
    tube demonstrated. Effect of magnet upon glow in small phosphorescent
    (powder) tubes examined. Marquis of Salisbury’s experiments
    (lighting-up with one wire only) tested, and confirmatory results arrived at.</p>

    <p>Chapter XVIII. Action of magnet on aura of electric spark demonstrated.</p>

    <p>Chapter XIX. Effects of magnet on discharges <i>in vacuo</i> in larger vessels
    demonstrated. Ångström’s flask experiment tested; same results not obtained
    unless one wire only was connected. Experiments demonstrating the action
    of a magnet on an electric stream, viz. vibration between, and rotation round,
    poles. Baron Reichenbach’s magnetic flames tested without result.</p>

  </section>

  <!-- Chapitre 20_________________________________________________________ -->
  <section class="chapter" id="chapter-20">
    <h3 class="titlechapter" id="chap-20">Some concluding Remarks</h3>
    <p class="shorter">Some concluding Remarks</p>


    <p>It is usual, in concluding a work on a special subject, to sum up its contents,
    and to examine the general results arrived at. This, however, it is not easy
    to do in the present case. The contents of our volume comprise a short
    history of the Aurora, its qualities and spectrum; and a statement has been
    given of the several conclusions at which various observers have arrived as to
    its character and causes. In the present state of our knowledge of the subject,
    to add an opinion to these might seem to savour of presumption; and the
    questions involved may perhaps be better treated as still <i>sub judice</i>, and as
    requiring further and fuller evidence before arriving at a verdict. The
    following observations must therefore be taken rather as further notes and
    memoranda, than as conclusions. Apart from the spectroscopic questions
    involved, the oldest and most received theory of the Aurora—that of its being
    some form of electric discharge in the more rarefied regions of the atmosphere,—seems
    to hold its own: and if, as is probable, some form of phosphorescence
    is involved in the discharge, M. Lecoq de Boisbaudran’s observations
    on the brightening of the red line under the influence of cold, and the falling
    of the yellow-green line within a band of phosphoretted hydrogen, come into
    play; and a connexion, though slight and imperfect, may be in this respect
    traced between the discharge and its spectrum. The experiments detailed in
    Part II. seem to have an important bearing, as showing the very marked
    effect of the magnet on the rarefied glow, as well as on the spark in air at
    ordinary pressure. The well-defined arc formed by the aura of the spark, the
    flickering jets which replace the even edge of the arc when partially withdrawn
    from the magnetic influence, and the streamers formed when the aura
    is blown away from the spark (Plate XVII. figs. 6, 7, and 8), are certainly
    highly suggestive of frequent forms of Auroral discharge; and, but for trial
    and failure, might lead one to expect results from a comparison of the line
    air-spectrum with that of the Aurora. The experiments with a wire attached
    to one electrode only, show how the glow may be affected and varied in colour
    and character when the discharge is interrupted and incomplete. Differences
    in electric tension may also considerably vary the character of the discharge.</p>

    <p></p>

    <p>The influence of the magnet in exciting and brightening the glow and
    spectrum of one gas, while it depresses and extinguishes the glow and spectrum
    of another gas in the same tube, suggests an explanation of the observed
    variation in intensity, and difference in number, of the Aurora-lines.
    Intensity of lines depending on temperature, and this again on resistance,
    and it appearing that resistance is influenced by the magnetic action, the
    same effects of brightening or depressing of the spectrum are probably produced
    in the Aurora, as in the vacuum-tubes placed between the magnet-poles.</p>

    <p>In the Marquis of Salisbury’s observations, paraffin-vapour gave C and H
    lines when connected with both poles of the battery, but C lines only when
    connected with one pole; and in that case the lines were equally sharp on
    both sides. These observations (repeated in our experiments) may afford an
    explanation why the hydrogen-lines are not seen in the Aurora-spectrum;
    although there can be hardly any doubt that the phenomenon usually takes
    place in air more or less moist. Professor Ångström’s researches on the violet-pole
    glow are not entirely corroborated by our experiments; and it seems
    doubtful whether his results in the exhausted flask were not obtained from
    the negative pole only. One great difficulty in the comparison of the Aurora-spectrum
    with the violet pole of air-tubes and some other spectra (including
    oxygen), arises from the presence in the latter of broad bands; and it is difficult
    to understand how these bands can be aptly compared with the definite,
    though faint, lines observed by Dr. Vogel and others in the Aurora-spectrum.
    It must, too, be borne in mind that the conditions under which we may consider
    the Aurora to obtain, are such as can be only very imperfectly imitated
    in the laboratory. Auroræ also no doubt differ in density and thickness of
    layer; and Kirchhoff’s observation must be remembered:—“That if thickness
    of a film of vapour be increased, the lines are increased in intensity, the
    bright lines more slowly than the fainter; and it may happen that the spectrum
    appears to be totally changed when the mass of the vapour is altered.”
    Were it possible to test with the spectroscope a cloud or film of gaseous
    vapour corresponding in some degree in density and thickness with an
    Auroral discharge, we might perhaps get nearer the truth. Mr. Procter also
    remarks (as we proved in our magnet experiments):—“That frequently very
    small traces appropriate to themselves the whole of electrical discharges at
    low pressures, and completely mask the spectra of any other gases present.”
    The oxygen-spectrum, with its possible variation by the conversion of that
    gas into the allotropic condition termed ozone, seemed at first to afford a
    prospect of close relation to the Aurora-spectrum; which, however, disappeared
    on closer examination. If nitrogen could be modified in some such
    way as oxygen is converted into ozone, it might perhaps afford another opportunity
    for investigation; but we have no evidence at present of such a change.
    The spectrum of nitrogen is usually found singularly distinct and persistent;
    and, except as varied from band to line by intensity of the discharge, not
    liable to alteration&nbsp;<span class="footnote">Dr. Schuster has found that while the line-spectrum of lightning is attributable to N, it has
  also a band-spectrum, which he considers due to O and a slight trace of CO₂ (Phil. Mag. 5th ser.
  vol. vii. p. 321).</span>.</p>

    <p>Colours of lines are functions of wave-length, subject, however, to the
    observation that in a weak spectrum the colours lose their intensity. The
    red line in the Aurora has sometimes been found brighter than the green.
    It has been suggested that the red and green may be independent spectra;
    but the variations of tint observed in the capillary of hydrogen and other
    tubes according to resistance of the current, demonstrate that the varying
    colours of the Aurora may be connected with the lighting-up of particular
    parts of the spectrum, and do not necessarily indicate that different gases and
    spectra are excited.</p>

    <p>Absorption may also play an important part in the nature of the Aurora-spectrum
    (Zöllner’s theory that the lines are really spaces between absorption
    bands). Most gases will give a continuous spectrum under certain circumstances,
    even at a low pressure.</p>

    <p>The question of cosmic dust is inviting, but the facts collated hardly
    warrant at present its probable connexion with the Aurora.</p>

    <p>If Auroræ were composed of incandescent glowing meteors, it would be
    reasonable to expect to find in the spectrum the lines of iron, a metal constituting
    so prominently the composition of meteorites. No connexion between
    the iron and the Aurora-spectra is, however, proved; though it may be
    suspected. The iron-spectrum, as remarked elsewhere, contains so many lines
    that some may, as a mere accidental circumstance, closely agree with the
    Aurora-lines.</p>

    <p>The iron-lines are, it may be remarked, as a rule, sharper and finer than
    the Auroral lines, though it is possible that these characteristics might vary
    if the spectrum were obtained in a rarefied medium. Tubes with iron terminals
    are said to evolve a compound gas of H and Fe. I have not had an
    opportunity to verify this.</p>

    <p></p>

    <p>It may be added that the comparative faintness of the more refrangible
    lines of the Aurora-spectrum suggests a feeble resistance to the exciting current,
    and a low temperature inconsistent with a meteoric theory; and this is
    not contradicted by the brightness of the red and green lines, if these are due
    to a phosphorescent origin. Expansion of a line is recognized to be dependent
    on pressure, and consequently the breadth of the green or red lines might
    indicate the height of the Aurora; while their brightness or otherwise might
    also give some idea as to its density. No observations in this direction have,
    as far as I am aware, been recorded.</p>

    <p>As the general result of spectrum work on the Aurora up to the present
    time, we seem to have quite failed in finding any spectrum which, as to position,
    intensity, and general character of lines, well coincides with that of the
    Aurora. Indeed, we may say we do not find any spectrum so nearly allied to
    portions even of the Aurora-spectrum, as to lead us to conclude that we have
    discovered the true nature of one spectrum of the Aurora (supposing it to
    comprise, as some consider, two or more). The whole subject may be characterized
    as still a scientific mystery—which, however, we may hope some future
    observers, armed with spectroscopes of large aperture and low dispersion, but
    with sufficient means of measurement of line positions, and possibly aided by
    photography, may help to solve. The singular absence of Auroræ has, for
    some time past, given no opportunity in that direction. May some of my
    readers be more fortunate in obtaining opportunities of viewing the glorious
    sky-fires, and assist to unravel so interesting a paradox!</p>

  </section>


  <section id="appendices">
  <h2 id="part-app">Appendices</h2>

      <!-- Appendix A_________________________________________________________ -->


      <section class="appendix" id="appendix-a">

        <h3 class="titleapp" id="app-a">References to some works and essays on the aurora</h3>
        <p class="shorter">References to some works and essays on the aurora</p>



        <p class="note-titre">(Most of these are cited in the ‘Edinburgh Encyclopædia’ and the ‘Encyclopædia Britannica.’)</p>

        <div class="hanging">

        <p>Musschenbroek, Instit. Phys. c. 41.</p>

        <p>‘Trai. Phys. et Hist. de l’Aurore Boréale,’ par M. de Mairan. Paris, 1754.</p>

        <p>Beccaria, ‘Dell’Elettricismo Artif. e Nat.’ p. 221.</p>

        <p>Smith’s ‘Optics,’ p. 69.</p>

        <p>D’Alembert’s ‘Opuscules Mathématiques,’ vol. vi. p. 334.</p>

        <p>‘Philosophical Transactions’ as under:—</p>

        <div></div>

        <table summary="References in Philosophical Transactions" id="table-appa-01">
          <thead>
            <tr>
              <th>Vol.</th>
              <th>Pages</th>
            </tr>
          </thead>
          <tr>
            <td>1716</td>
            <td>406</td>
          </tr>
          <tr>
            <td>1717</td>
            <td>584, 586</td>
          </tr>
          <tr>
            <td>1719</td>
            <td>1099, 1101, 1104, 1107</td>
          </tr>
          <tr>
            <td>1720</td>
            <td>21</td>
          </tr>
          <tr>
            <td>1721</td>
            <td>180, 186</td>
          </tr>
          <tr>
            <td>1723</td>
            <td>300</td>
          </tr>
          <tr>
            <td>1724</td>
            <td>175</td>
          </tr>
          <tr>
            <td>1726</td>
            <td>128, 132, 150</td>
          </tr>
          <tr>
            <td>1727</td>
            <td>245, 301</td>
          </tr>
          <tr>
            <td>1728</td>
            <td>453</td>
          </tr>
          <tr>
            <td>1729</td>
            <td>137</td>
          </tr>
          <tr>
            <td>1730</td>
            <td>279</td>
          </tr>
          <tr>
            <td>1731</td>
            <td>53-55</td>
          </tr>
          <tr>
            <td>1734</td>
            <td>243, 291</td>
          </tr>
          <tr>
            <td>1736</td>
            <td>241</td>
          </tr>
          <tr>
            <td>1740</td>
            <td>368</td>
          </tr>
          <tr>
            <td>1741</td>
            <td>744, 839, 840, 843</td>
          </tr>
          <tr>
            <td>1750</td>
            <td>319, 345, 346, 499</td>
          </tr>
          <tr>
            <td>1751</td>
            <td>39, 126</td>
          </tr>
          <tr>
            <td>1752</td>
            <td>169</td>
          </tr>
          <tr>
            <td>1753</td>
            <td>85</td>
          </tr>
          <tr>
            <td>1762</td>
            <td>474, 479</td>
          </tr>
          <tr>
            <td>1764</td>
            <td>326, 332</td>
          </tr>
          <tr>
            <td>1767</td>
            <td>108</td>
          </tr>
          <tr>
            <td>1769</td>
            <td>86, 307</td>
          </tr>
          <tr>
            <td>1770</td>
            <td>532</td>
          </tr>
          <tr>
            <td>1774</td>
            <td>128</td>
          </tr>
          <tr>
            <td>1781</td>
            <td>228</td>
          </tr>
          <tr>
            <td>1790</td>
            <td>32, 47, 101</td>
          </tr>
        </table>

        <p></p>

        <p>‘Miscell. Berolinens.’ 1710, vol. i. p. 131.</p>

        <p>‘Comment. Petrop.’ tom. i. p. 351, tom. iv. p. 121.</p>

        <p>‘Acta Petrop.’ 1780, vol. iv. p. 1.</p>

        <p>‘Mem. Acad. Paris,’ 1747, pp. 363, 423; 1731; 1751.</p>

        <p>‘Mem. Acad. Berl.’ 1710, vol. i. p. 131; 1747, p. 117.</p>

        <p>Schwed. ‘Abhandlungen,’ 1752, p. 169; 1753, p. 85; 1764, pp. 200, 251.</p>

        <p>Bergman, ‘Opusc.’ vol. v. p. 272.</p>

        <p>‘Americ. Trans.’ vol. i. p. 404.</p>

        <p>‘Mém. de Mathémat. et Phys.’ tom. viii. p. 180.</p>

        <p>Rozier, vol. xiii. p. 409; vol. xv. p. 128; vol. xxxiii. p. 153.</p>

        <p>Franklin’s Works, vol. ii.</p>

        <p>Weidler, ‘De Aurora Boreale.’ 4to.</p>

        <p>Nocetus, ‘De Iride et Aurora Boreale, cum Notis Boscovisch.’ Rome, 1747.</p>

        <p>Chiminello, ‘Mem. Soc. Ital.’ vol. vii. p. 153.</p>

        <p>Gilbert’s ‘Journal,’ vol. xv. p. 206; and (particularly) Dr. T. Young’s ‘Nat.
        Phil.’ vol. i. pp. 687, 716, and vol. ii. p. 488.</p>

        <p>Wiedeburg, ‘Ueber die Nordlichter.’ Jena, 1771.</p>

        <p>Hüpsch, ‘Untersuchung des Nordlichts.’ Cologne, 1778.</p>

        <p>Van Swinden, ‘Recueil de Mémoires.’ Hague, 1784.</p>

        <p>Wilke, ‘Von den neuesten Erklärungen des Nordlichts,’ Schwed. Mus.
        Wismar, 1783.</p>

        <p>Dalton’s ‘Meteor. Observ.’ 1793, pp. 54, 153.</p>

        <p>Loomis, ‘Sill. Journal,’ 2nd series, xxxii. p. 324; xxxiv. p. 34. The same,
        3rd series, v. p. 245; B. V. Marsh, 3rd series, xxxi. p. 311.</p>

        <p>Oettingen and Vogel, Pogg. Ann. cxlvi. pp. 284, 569.</p>

        <p>Galle and Sirks, <i>ibid.</i> cxlvi. p. 133; cxlix. p. 112.</p>

        <p>Silbermann, ‘Comptes Rendus,’ lxviii. pp. 1049, 1120, 1140, 1164.</p>

        <p>Prof. Fritz, “Geog. Distrib.,” Petermann’s Mitth., Oct. 1874.</p>

        <p>Zehfuss, ‘Physikalische Theorie.’ Adelman, Frankfort.</p>

        <p>‘Nature,’ iii. pp. 6, 7, 28, 104, 126, 346, 348, 510; iv. pp. 209, 213, 345, 497,
        505; x. 211 (Ångström).</p>

        <p>‘Edinburgh Astronomical Observations,’ vol. xiv. 1870-1877.</p>

        <p>‘English Mechanic,’ No. 461 (January 23, 1874), pp. 445-447; and No. 462,
        pp. 475, 476.</p>

        </div>

      </section>

      <!-- Appendix B_________________________________________________________ -->
      <section class="appendix" id="appendix-b">

        <h3 class="titleapp" id="app-b">Extracts from the manual and instructions for the (english) arctic expedition, 1875</h3>
        <p class="shorter">Extracts from the manual and instructions for the (english) arctic expedition, 1875</p>




        <h4><i>Note on Auroral Observations. By Prof. <span class="smcap">Stokes</span>, Sec. R.S.</i></h4>

        <p>The frequency of the Aurora in Arctic regions affords peculiar facilities for
        the study of the general features of the phenomenon, as in case the observer
        thinks he has perceived any law he will probably soon, and repeatedly, have
        opportunities of confronting it with observation. The following points are
        worthy of attention:—</p>

        <p><i>Streamers.</i>—It is well known that, at least as a rule, the streamers are
        parallel to the dipping-needle, as is inferred from the observation that they
        form arcs of great circles passing through the magnetic zenith. It has been
        stated, however, that they have sometimes been seen curved. Should any thing
        of this kind be noticed, the observer ought to note the circumstances most
        carefully. He should notice particularly whether it is one and the same
        streamer that is curved, or whether the curvature is apparent only, and arises
        from the circumstance that a number of short, straight streamers start from
        bases so arranged that the luminosity as a whole presents the form of a curved
        band.</p>

        <p>Have the streamers any lateral motion? and if so, is it from right to left or
        left to right, or sometimes one and sometimes the other, according to the
        quarter of the heavens in which the streamer is seen, or other circumstances?
        Again, if there be lateral motion, is it that the individual streamers move
        sideways, or that fresh streamers arise to one side of the former, or partly the
        one and partly the other? Do streamers, or does some portion of a system of
        streamers, appear to have any uniform relation to clouds, as if they sprang
        from them? Can stars be seen immediately under the base of streamers? Do
        streamers appear to have any definite relation to mountains? Are they ever
        seen between the observer and a mountain, so as to appear to be projected on
        it? This or any other indication of a low origin ought to be most carefully
        described.</p>

        <p>When streamers form a corona, the character of it should be described.</p>

        <p><i>Auroral Arches.</i>—Are arches always perpendicular to the magnetic meridian?
        If incomplete, do they grow laterally? and if so, in what manner, and
        towards which side? Do they always move from north (magnetic) to south?
        and if so, is it by a southerly motion of the individual streamers, or by new
        streamers springing up to the south of the old ones? What (by estimation,
        or by reference to known stars) may be the breadth of the arch in different
        positions in its progress? Do arches appear to be nothing but congeries of
        streamers, or to have an independent existence? What relations, if any, have
        they to clouds? and if related, to what kind of clouds are they related?</p>

        <p><i>Pulsations.</i>—Do pulsations travel in any invariable direction? What time
        do they take to get from one part of the heavens to another? Are they running
        sheets of continuous light, or fixed patches which become luminous, or
        more luminous, in rapid succession? and if patches, do these appear to be
        foreshortened streamers? Are the same patches luminous in successive
        pulsations?</p>

        <p><i>Sounds</i> (?).—As some have suspected the Aurora to be accompanied by
        sound, the observer’s attention should be directed to this question when an
        Aurora is seen during a calm. If sound be suspected, the observer should
        endeavour, by changing his position, brushing off spicules of ice from the
        neighbourhood of the ears, his whiskers, &amp;c., to ascertain whether it can be
        referred to the action of such wind as there is on some part of his dress or
        person. If it should clearly appear that it is not referable to the wind, then
        the circumstance of its occurrence, its character, its relation (if any) to bursts
        of light, should be most carefully noted.</p>

        <p>These questions are prepared merely to lead the observer to direct his
        attention to various features of the phenomenon. Answers are not demanded,
        except in such cases as definite answers can be given; and the observer should
        keep his attention alive to observe and regard any other features which may
        appear to be of interest. It is desirable that drawings should be made of
        remarkable displays.</p>

        <p>Observations with Sir William Thomson’s electrometer would be very
        interesting in connexion with the Aurora, especially a comparison of the
        readings before, during, and after a passage of the Aurora across the zenith.</p>

        <p></p>

        <h4><i>Spectroscopic Observations. By Prof. <span class="smcap">G. G. Stokes</span>, Sec. R.S. <br/>Spectrum of the Aurora.</i></h4>


        <p>The spectrum of the Aurora contains a well-known conspicuous bright line
        in the yellowish green, which has been accurately observed. There are also
        other bright lines of greater refrangibility, the determination of the positions
        of which is more difficult on account of their faintness, and there are also one
        or more lines in the red, in red auroras.</p>

        <p>Advantage should be taken of an unusually bright display to determine the
        positions of the fainter lines. That of the brightest lines, though well known,
        should be measured at the same time to control the observations. The character
        of the lines (<i>i. e.</i> whether they are strictly lines, showing images of the
        apparent breadth of the slit, or narrow bands, sharply defined or shaded-off)
        should also be stated.</p>

        <p>Sometimes a faint gleam of light is seen at night in the sky, the origin of
        which (supposed from the presence of clouds) is doubtful. A spectroscope of
        the roughest description may in such cases be usefully employed to determine
        whether the light is auroral or not, as in the former case the auroral
        origin is detected by the chief bright line. The observer may thus be led
        to be on the look-out for a display which otherwise might have been
        missed.</p>

        <p>It has been said, however, that the auroral light does not in all cases exhibit
        bright lines, but sometimes, at least in the eastern and western arch of the
        Aurora, shows a continuous spectrum. This statement should be confronted
        with observation, special care being taken that the auroral light be not confounded
        with light which, though seen in the same direction, is of a different
        origin, such, for example, as light from a bank of haze illuminated by the
        moon.</p>

        <p>Sir Edward Sabine once observed an auroral arch to one side (say north) of
        the ship, which was in darkness. Presently the arch could no longer be seen,
        but there was a general diffuse light, so that a man at the mast-head could be
        seen. Later still, the ship was again in darkness, and an auroral arch was
        seen to the south.</p>

        <p>Should any thing of the kind be observed, the whole of the circumstances
        ought to be carefully noted, and the spectroscope applied to the diffuse
        light.</p>

        <p></p>

        <h4><i>Polarization of Light. By <span class="smcap">W. Spottiswoode</span>, M.A., LL.D., Treas. R.S.</i></h4>

        <p>It has been suggested that the Aurora, inasmuch as it presents a structural
        character, may afford traces of polarization. Having reference to the fact that
        the striæ of the electric discharge in vacuum-tubes present no such feature, the
        probability of the suggestion may be doubted. But it will still be worth while
        to put the question to an experimental test.</p>

        <p>If traces of polarization be detected, it must not at once be concluded that
        the light of the Aurora is polarized; for the Aurora may be seen on the background
        of a sky illuminated by the moon, or by the sun, if not too far below
        the horizon, and the light from either of these sources is, in general, more or
        less polarized; therefore, if the light of the Aurora is suspected to be polarized,
        the polariscope should be directed to an adjacent portion of clear sky, free
        from Aurora, but illuminated by the moon or sun as nearly as possible
        similar, and similarly situated to the former portion; and the observer must
        then judge whether the polarization first observed be merely due to the illumination
        of the sky.</p>

        <p>The presence of polarization is to be determined:—</p>

        <p>(1) With a Nicol’s prism, by observing the light through it by turning the
        prism round on its axis, and by examining whether the light appears brightest
        in some positions and least bright in others. If such be the case, the positions
        will be found to be at right angles to one another. The direction of
        “the plane of polarization” will be determined by that of the Nicol at either
        of these critical positions. The plane of polarization of the light transmitted
        by a Nicol, is parallel to the longer diagonal of the face; and, accordingly, the
        plane of polarization, or partial polarization, of the observed light is parallel
        to the longer diameter of the Nicol when the transmitted light is at its greatest
        intensity, or to the shorter when it is at its least.</p>

        <p>(2) The observation with a double-image prism is similar to that with a
        Nicol. This instrument, as its name implies, gives the images which would
        be seen through the Nicol in two rectangular positions, both at once, so that
        they can be directly compared; and when in observing polarized light the
        instrument is turned so that one image is at a maximum, the other is
        simultaneously at a minimum. Both these methods of observation, (1) and
        (2), are especially suitable for faint light; because in such a case the eye is
        better able to appreciate differences of intensity than differences of colour.</p>

        <p>(3) The observation with a biquartz differs from (1) only by holding a biquartz
        (a right-handed and a left-handed quartz cemented side by side) at a
        convenient distance beyond the Nicol, and by observing whether colour is or
        is not produced. If the Nicol be so turned that the two parts of the biquartz
        give the same colour (choose the neutral tint, <i>teint de passage</i>, rather than the
        yellow), we can detect a change in the position of the plane of polarization by
        a change in colour, one half verging towards red, the other towards blue. This
        observation is obviously applicable to a change in the plane, either at different
        parts of the phenomenon at the same time, or at the same parts at different
        times.</p>

        <p>(4) We may use a Savart’s polariscope, which shows a series of coloured
        bands in the field of view. For two positions at right angles to one another
        corresponding to the two critical positions of a Nicol, these bands are most
        strongly developed; for two positions midway between the former the bands
        vanish. In the instruments here furnished, the plane of polarization of the
        observed light will be parallel to the bands when the central one is light, perpendicular
        to them when the central band is dark.</p>

        <h4><i>Instructions in the use of the Spectroscopes supplied to the Arctic Expedition.</i>
        <i>By <span class="smcap">J. Norman Lockyer</span>, F.R.S.</i><br/><i>Spectroscopic Work.</i></h4>

        <p>Scales prepared on Mr. Capron’s plan, together with forms for recording
        positions, also accompany the instrument.</p>

        <p>In using these, carefully insert the principal solar lines in their places
        on the forms, as taken from a fine slit, and keep copies of this scale for use.
        If the slit opens <i>only on one</i> side, note on scale in which direction the lines
        widen out, whether towards red or violet. Also fill up some of these forms
        with gas and other spectra, as taken at leisure <i>with the same instrument</i> and
        scale.</p>

        <p>When observing, close the slit (after first wide opening it) as much as
        light will permit, and then with pen or pencil record the lines as seen upon
        the micrometer-scale on the corresponding part of the form, and note <i>at once
        relative intensities</i> with Greek letters, α, β, &amp;c. (or numbers).</p>

        <p>Reduce at leisure line-places on scale to wave-lengths, and note as to each
        line the <i>probable limits of instrumental error</i>.</p>

        <p>In case the auroral spectrum is so faint that the needle-point or micrometer-scale
        is invisible, half of the field of view may be covered with tinfoil,
        with a perfectly straight smooth edge running along the diameter of the field,
        in perfect focus, and parallel to the lines of the spectra. The reading-screw
        being set to 10, the bending-screw should then be adjusted so that the green
        line of the Aurora is just eclipsed behind the blackened edge of the tinfoil. A
        similar eclipse of other lines will give their positions.</p>

        <p>In this instrument the reference-prism is brought into action by turning the
        slipping piece to which is fixed the two terminals. Care should be taken that
        the prism itself is adjusted before commencing observations, as it may be shaken
        out of position on the voyage. The tubes provided for the reference-spectra
        may be either fastened to the terminals or arranged in some other manner.
        The air-spectrum may also be used as a reference-spectrum. To get this,
        two wires should be screwed into the insulators, their ends being at such
        a distance apart and in such a position that the spectrum is well seen.</p>

        <h4><i>General Observations regarding the Spectrum of the Aurora</i>&nbsp;<span class="footnote">In these observations some suggestions made by Mr. Capron have been incorporated.</i><br/>[This was Mr. Lockyer’s note. In point of fact, the Author was responsible for the verbatim
      paragraphs comprised between the letters A and B, and C and D, in the instructions as now
      reprinted.]</span></h4>

        <p>Note appearance, colour, &amp;c. of <i>arc</i>, <i>streamers</i>, <i>corona</i>, and <i>patches</i> of light.</p>

        <p>Get compass positions of principal features, and <i>note any change of magnetic
        intensity</i>. If corona forms, take its position and apparent height.</p>

        <p>Look out for <i>phosphorescence</i> of Aurora and adjacent clouds. Listen for
        reported sounds. Note any peculiarity of cloud scenery, prior to or pending
        the Aurora.</p>

        <p>Sketch principal features of the display, and indicate on this sketch the
        parts spectroscopically examined.</p>

        <p>Examine line in <i>red</i> specially in reference to its assumed connexion with
        <i>telluric</i> lines (little <b>a</b> group), and note <i>as to its brightening in sympathy with
        any of the other lines</i>.</p>

        <p>Examine line in yellow-green (Ångström’s) as to <i>brightness</i>, <i>width</i>, and
        <i>sharpness</i> (<i>or nebulosity</i>) at the edges. Notice as to a peculiar <i>flickering</i> in this
        line sometimes seen; note also whether this line is <i>brighter</i> (or the reverse)
        <i>with a fall of temperature</i>. Note <i>ozone</i> papers at the time of Aurora.</p>

        <p>Note whether the Auroræ can by their spectra be classed into distinct types
        or forms, and examine for <i>different spectra</i> as under:—</p>


        <p>α. The auroral <i>glow</i>, pure and simple.</p>

        <p>β. The white arc.</p>

        <p>γ. The streamers and corona.</p>

        <p>δ. Any phosphorescent or other patches of light, or light cloud in or
        near the Auroræ.</p>


        <p>The information collected together in the ‘Manual’ should be carefully
        consulted, and the line of observations suggested by Ångström’s later work
        followed out. To do this, not only record the positions of any features you
        may observe in the spectrum, but endeavour to determine, if any, and if so
        which, of the features vary together. Compare, for instance, the two spectra
        of nitrogen in the Geissler tube supplied, by observing first the narrow and
        then the wider parts of the tube. It will be seen that the difference in colour
        and spectrum results simply from an addition to the spectrum in the shape of
        a series of channelled spaces in the more refrangible end in the case of the
        spectrum of the narrow portion.</p>

        <p>Try to determine whether the difference between red and green Auroras
        may arise from such a cause as this, and which class has the simpler
        spectrum.</p>

        <p>See whether indications of great auroral activity are associated with the
        widening or increased brilliancy of any of the auroral lines.</p>

        <p>Remember that if auroral displays are due to gaseous particles thrown into
        vibration of electric disturbance, increased electric tension may either (1) dissociate
        those particles and thus give rise to a new spectrum, the one previously
        observed becoming dimmer; or (2) throw the particles into more intense
        vibration without dissociation, and thus give rise to new lines, those previously
        observed becoming brighter.</p>

        <p>Careful records of auroral phenomena from both ships may enable the
        height of some, observed from both, to be determined. It will be very important
        that those the heights of which are determined by such means should be
        carefully observed by the spectroscope, in order to observe whether certain
        characteristics of the spectrum can be associated with the height of the
        Aurora.</p>

      </section>

      <!-- Appendix C_________________________________________________________ -->
      <section class="appendix" id="appendix-c">

        <h3 class="titleapp" id="app-c">
          Extracts from parliamentary blue book, containing the “Results derived from the arctic expedition 1875-76” (Eyre and Spottiswoode, 1878)
        </h3>
        <p class="shorter">Extracts from parliamentary blue book</p>




        <h4><i>Auroras observed 1875-1876, at Floebery Beach and Discovery Bay. By Lieutenant <span class="smcap">A. C. Parr</span>, R.N.</i></h4>


        <p>Though the auroral glow was often present, and served in some degree
        to lighten the darkness of the sky during the long winter, when the moon
        was absent, the actual appearances of the Aurora itself were few, and the
        nimbus worthy of any particular remark extremely small. Those which
        were stationary assumed the form of low arches, with streamers flashing
        up to them from the horizon, and usually to the eastward. But the more
        common form was for an arch to appear low down in some part of the sky
        where the glow was brightest; at first it was very faint and narrow, but as it
        rose gradually in the heavens it would increase both in size and intensity, till
        on arriving near the zenith, with its ends extending nearly to the horizon, it
        would be about the breadth of three or four rainbows, and its colour that of
        white fleecy clouds lit up by the rays of the full moon. On reaching this
        point, however, its course was nearly run; for after appearing to remain
        stationary, as little white gaps would suddenly rend the arch asunder, the
        portions thus detached seemed to roll together and concentrate all their
        brightness in the smaller space, and then gradually fade away and become
        extinct. Sometimes a very pale green would show itself in the more luminous
        patches, and once or twice there was a slight suspicion of red; but never was
        the whole sky illuminated by streams running in all directions, and forming
        coronæ, while these colours varied every moment.</p>

        <p>When instead of the arch rising up from the horizon a streamer appeared,
        its origin was in the north. From the northern horizon it would stretch out
        towards the zenith, passing nearly overhead, and reaching to within a few
        degrees of the land to the south. In appearance they would be the same as
        the arches, but sometimes a second would grow out of the first, and on one
        occasion three were visible at the same time. They had lateral motion either
        from east to west, or west to east, but there was no flashing to brighten them,
        and they gradually faded away.</p>

        <p>The time at which Auroras usually occurred was between 9 <span class="smcapuc">P.M.</span> and midnight,
        the last display being on February 19th, commencing at 11 <span class="smcapuc">P.M.</span> It
        was a beautifully clear night, without mist or haze of any description, and
        small stars visible close down to the horizon. At the above-named hour two
        arches made their appearance, and remained stationary; the lower one was
        the brighter, being of a pale green colour, its centre bearing E.S.E. (true),
        and having an altitude of about 5°, with a breadth of about twice that of a
        rainbow. The second arch was concentric with the first, and about 7° above
        it, but rather broader and fainter. These arches maintained their altitude,
        the upper one at about the same intensity, but that of the lower one varied
        considerably. It would gradually lighten up, then send flashes to the upper
        one, then break up and fade away; before, however, it had quite disappeared,
        flashes would come up to it from the horizon which seemed to endue it with
        new life, for the arch would be reformed, brighten up, and the same performance
        would be again repeated. This occurred three or four times in the
        course of three quarters of an hour; but the flashes from the horizon never
        extended beyond the lower arch, and those from the lower never went beyond
        the upper. During this display the citron-line was obtained very clearly
        with the spectroscope, but no other lines were visible.</p>

        <p>On six or seven occasions Auroras were visible at the same time on board
        both the ‘Alert’ and ‘Discovery;’ but the absence of characteristic features
        makes it impossible to determine whether they were the same display, or
        merely two distinct ones which happened to occur at the same time. But as
        by far the larger number of those recorded in the one ship were not visible
        at the other, it was certainly only under exceptional conditions that they
        could be simultaneously observed at both stations, if, indeed, they ever were.
        Auroras seemed to appear indifferently both when there was wind and when
        it was calm, with either a high or low barometer, and seemed quite unconnected
        with the temperature, although on an occasion the thermometer was
        observed to fall 3° during the display, and to rise 2° almost immediately afterwards.
        But it was never seen illuminating the edges of clouds, as we saw it
        on the passage home, nor playing about the outline of the land, and never
        was there the slightest suspicion of sound being produced by it.</p>

        <p></p>

        <p>The opportunities for observing the spectrum of the Aurora in this position
        have been most unsatisfactory, as the displays were small in number and
        deficient in brilliancy.</p>

        <p>The form they generally assumed was to rise like an arch from a portion
        of the horizon where there was a luminous glow, at first very faint, but gradually
        increasing in brilliancy till near the zenith, where it would remain
        stationary for a short time and then break up and disappear. Sometimes
        they would rise up as streamers, but only occasionally was more than one
        visible at a time, and they lasted for such a short time, that even if they had
        been bright it would have been very difficult to make satisfactory observations.</p>

        <p>Very few showed any signs of colour, and those only the slightest tinge.
        Nearly all that were observed gave the citron-line with the small pocket
        spectroscope with more or less distinctness, though no signs of any other
        lines were ever seen; but on only two occasions was it bright enough to get
        the line with Nury’s spectroscope, and then only for such a short time that
        a satisfactory measure could not be obtained.</p>

        <p class="separator"></p>

        <p>Then follows a descriptive list of the Auroræ seen, from which I have
        selected three of the finest, viz. January 2nd, February 14th, and February
        19th, 1876.</p>

        <p>January 2nd, 1876. Lieut. Parr. <i>Floeberg Beach.</i>—9 <span class="smcapuc">P.M.</span> Streams of
        Aurora. Stars shining brightly.</p>

        <p>Register. <i>Discovery Bay.</i>—9 <span class="smcapuc">P.M.</span> Observed an Aurora like a pale band
        of light in the form of an arch whose centre was on the true meridian and
        15° from the zenith. It shortly afterwards broke up into feathered edges,
        their direction being a little to the eastward of the zenith. The arch grew
        fainter, and shifted to the eastward of the meridian four points; the left
        extremity of the arch faded away, and the right assumed the shape of the
        folds of a curtain doubled over. The weather was clear and calm. The display
        lasted upwards of 30 minutes.</p>

        <p>A spectroscope, one of Browning’s 8-in. direct-vision, was directed towards
        the Aurora, but the light was not sufficient to give any spectrum.</p>

        <p>The temperature was -39°. Barometer 29·56 inches. No wind. Clouds
        stratus 2. Eight meteors were observed during the time the Aurora was
        visible.</p>

        <p>February 14th. Register. <i>Discovery Bay.</i>—At 2 <span class="smcapuc">A.M.</span> a faint Aurora
        passing across the heavens from S.E. to S.W. was observed, like an arch of
        a pale colour. It lasted only a short time, and was very indistinct. Temperature
        -47°. Barometer 30·44 inches. No wind or clouds.</p>

        <p>Lieut. Aldrich. <i>Floeberg Beach.</i>—2 <span class="smcapuc">A.M.</span> A faint Aurora towards the
        S.W. Weather calm. Cumulus-stratus clouds 3. Temperature -46°.
        8 <span class="smcapuc">P.M.</span> Faint flashes of Aurora in the E. and S.W.</p>

        <p>Lieut. Aldrich and Lieut. Parr. <i>Floeberg Beach.</i>—11.50 <span class="smcapuc">P.M.</span> A moderately
        bright arch of Aurora extended from due N. to about S.S.W., where
        it terminated close down to the horizon in a crook turned to the eastward.
        In a few moments a streamer flashed from the end of the crook parallel to
        the first and right across the heavens, its edges being quite sharp and parallel
        to each other. A third streamer shot up a minute afterwards, but did not
        extend more than 80° upwards. The streamers were visible for a very short
        time, the first remaining longest. The second-named arch gradually faded
        away till within a few degrees of the S.S.W. horizon, and (still being a continuation
        of the crook) bent round to the eastward, and towards the horizon,
        going on to what was left of the stump of the third arc. A lateral motion
        to the eastward now began, the whole body gradually turning round until it
        disappeared about due south. Stars were visible through it at its brightest,
        but not very distinctly. This is the most intense and variegated Aurora we
        have experienced, but scarcely any colours were to be seen. Temperature
        -51°. Barometer 30·43 inches, stationary. Calm weather. Clouds cumulus
        1. Preceded and followed by calm weather.</p>

        <p>Meteorological Register. <i>Discovery Bay.</i>—9.15 <span class="smcapuc">P.M.</span> An Aurora was
        observed to the southward, spreading out like a fan in separate ways. It
        was faint. A few cirro-stratus clouds were visible, apparently between the
        observer and the Aurora. It lasted about 40 minutes, and then gradually
        faded away. Temperature -47°. Barometer 30·51 inches, stationary. No
        wind. Clouds cirro-stratus 4.</p>

        <p>February 19th. Meteorological Report. <i>Discovery Bay.</i>—9.45 <span class="smcapuc">P.M.</span> An
        Aurora like a fluted arch, with rays flashing towards the Pole, was observed
        spanning the hills from the south to the east. The direction of the lines of
        light from all parts of the arch was towards the zenith. Above the arch a
        pale band of colour appeared, like a secondary arch above the other. It
        appeared very much as if it was caused by the reflected light of the Aurora.
        The Aurora was bright for a few seconds, and then gradually died away. It
        lasted altogether about 30 minutes. The centre of the arch bore S.E., having
        an altitude of about 30°. The secondary arch was about 15° above the
        former. Both arches were of a pale light colour, the upper one very faint.
        Temperature -34°. Barometer 29·87 inches, rising rapidly. Weather calm.
        Misty. No clouds.</p>

        <p>Lieut. Parr. <i>Floeberg Beach.</i>—An Aurora appeared shortly after 11 P.M.,
        consisting of bright arch, whose centre bore about E.S.E., and had an altitude
        of about 5°, with a second broader and fainter arch about 7° above the
        first. These arches maintained their altitudes, the upper one at about the
        same intensity, but that of the lower one varied considerably. It would
        gradually brighten up, then send streamers up to the second, then break up
        into light patches, and gradually fade away. This happened three or four
        times during the 40 minutes that the display lasted. At times streamers
        would come up from the horizon to the lower arch, for it was a splendidly
        clear night, and seemed to brighten it up, but none of them extended beyond
        it. Neither did the streamers from the lower arch extend beyond the upper
        one. It was slightly green in colour when brightest, and the citron-line was
        well defined, but no others were visible. Temperature -46°. Barometer
        29·95 inches, steady. Weather calm. Cumulus clouds 4. Misty.</p>


        <table class="borders">
          <caption><span class="smcap">Table</span> of <span class="smcap">Dates</span> when <span class="smcap">Auroras</span> were observed by the <span class="smcap">Arctic Expedition</span>, 1875-76</caption>
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>H.M.S. ‘Alert,’ Floeberg Beach.</th>
              <th>H.M.S. ‘Discovery,’ Discovery Bay.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1875,</td>
            <td class="nobr">October</td>
            <td class="tdr">25</td>
            <td>11.45 <span class="smcapuc">P.M.</span> Faint.</td>
            <td>Cloudy.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">26</td>
            <td>10 <span class="smcapuc">P.M.</span> Very faint.</td>
            <td>10 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">30</td>
            <td>Sky obscured. Faint.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="nobr">November</td>
            <td class="tdr">1</td>
            <td>Ditto. Faint, but well marked.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">2</td>
            <td>9 to 10 <span class="smcapuc">P.M.</span> Arches and streamers.</td>
            <td>A few clouds.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">21</td>
            <td>Ditto. Bright streamer.</td>
            <td>9 to 10 <span class="smcapuc">P.M.</span> and 10 to 11 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">22</td>
            <td>2 <span class="smcapuc">P.M.</span> and 8 <span class="smcapuc">P.M.</span> Slight, red.</td>
            <td>Clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">25</td>
            <td>9.30 <span class="smcapuc">A.M.</span> Character not recorded.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">26</td>
            <td>10 <span class="smcapuc">A.M.</span> Stream of light.</td>
            <td rowspan="2"><span class="x2">}</span> A few clouds. 10 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">26</td>
            <td>Cloudy to 10 <span class="smcapuc">P.M.</span>, bright afterwards.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">27</td>
            <td>Midnight. Slight.</td>
            <td>11.40 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">28</td>
            <td>1 <span class="smcapuc">A.M.</span> Bright streak.</td>
            <td>Clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">29</td>
            <td>Cloudy, brighter at 11 <span class="smcapuc">A.M.</span> Faint glow.</td>
            <td>9.30 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">30</td>
            <td>A few clouds. Very faint.</td>
            <td>4.30 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">30</td>
            <td>5 <span class="smcapuc">P.M.</span>, 8 <span class="smcapuc">P.M.</span>, and 10 <span class="smcapuc">P.M.</span> Flashes.</td>
            <td>5 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="nobr">December</td>
            <td class="tdr">2</td>
            <td>Evening. Streamers.</td>
            <td>Clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">3</td>
            <td>1 <span class="smcapuc">A.M.</span> Flashes.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">3</td>
            <td>Bright sky. Faint Aurora.</td>
            <td>2.30 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">16</td>
            <td>10 <span class="smcapuc">P.M.</span> Slight; showed citron-line.</td>
            <td>11 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">19</td>
            <td>3 <span class="smcapuc">P.M.</span> to 5 <span class="smcapuc">P.M.</span>, faint; and 9 to 10 <span class="smcapuc">P.M.</span>, moderately bright arc.</td>
            <td>Very clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">22</td>
            <td>10 <span class="smcapuc">P.M.</span> Slight.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">23</td>
            <td>6 <span class="smcapuc">P.M.</span> Ditto.</td>
            <td>Ditto.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">24</td>
            <td>Misty, a few stars visible. Arch.</td>
            <td>9 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">26</td>
            <td>Very bright sky. Faint.</td>
            <td>6 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">29</td>
            <td>Ditto. Very faint.</td>
            <td>6.15 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">31</td>
            <td>4 <span class="smcapuc">P.M.</span> Same.</td>
            <td>Sky obscured.</td>
          </tr>
          <tr>
            <td class="bl nobr">1876,</td>
            <td class="nobr">January</td>
            <td class="tdr">1</td>
            <td>5 <span class="smcapuc">P.M.</span> and 11 <span class="smcapuc">P.M.</span> Slight.</td>
            <td>A few clouds.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">2</td>
            <td>9 <span class="smcapuc">P.M.</span> Described and figured.</td>
            <td>9 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">17</td>
            <td>Very bright sky. Very faint streamers.</td>
            <td>9.25 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">18</td>
            <td>9.45 <span class="smcapuc">P.M.</span> and 10.5 <span class="smcapuc">P.M.</span> Character not recorded.</td>
            <td>10.15 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">19</td>
            <td>Very bright sky. Faint.</td>
            <td>9.45 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">20</td>
            <td>2 <span class="smcapuc">A.M.</span> Slight.</td>
            <td>2.30 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">23</td>
            <td>7.55 <span class="smcapuc">A.M.</span> and 2 <span class="smcapuc">P.M.</span> Slight.</td>
            <td>8.45 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">24</td>
            <td>Bright sky. Slight flash.</td>
            <td>2 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">24</td>
            <td>5 <span class="smcapuc">P.M.</span> and 11.15 <span class="smcapuc">P.M.</span> Faint Aurora.</td>
            <td>Very clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">27</td>
            <td>2 <span class="smcapuc">A.M.</span> to 3.45 <span class="smcapuc">A.M.</span> Faint.</td>
            <td>1 <span class="smcapuc">A.M.</span> to 4 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">27</td>
            <td>Very bright sky. Faint double arch.</td>
            <td>8.30 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">28</td>
            <td>6 <span class="smcapuc">P.M.</span> and 7 to 9 <span class="smcapuc">P.M.</span> Faint flashes.</td>
            <td>7.20 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">30</td>
            <td>8 <span class="smcapuc">P.M.</span> Streak.</td>
            <td>7.50 to 9 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">31</td>
            <td>8.30 <span class="smcapuc">A.M.</span> and 7.30 <span class="smcapuc">P.M.</span> Very faint.</td>
            <td>8.25 <span class="smcapuc">A.M.</span>, 5.30 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="nobr">February</td>
            <td class="tdr">3</td>
            <td>10 <span class="smcapuc">P.M.</span> Slight flash.</td>
            <td>Very clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">11</td>
            <td>Sky obscured. Very faint.</td>
            <td>11 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">13</td>
            <td>11 <span class="smcapuc">P.M.</span> Flashes.</td>
            <td>Clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">14</td>
            <td>2 <span class="smcapuc">A.M.</span>, 9.15 to 10 <span class="smcapuc">P.M.</span> Described and figured.</td>
            <td>2 <span class="smcapuc">A.M.</span> and 11.50 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">19</td>
            <td>9.45 <span class="smcapuc">P.M.</span></td>
            <td>11 <span class="smcapuc">P.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">20</td>
            <td>2 <span class="smcapuc">A.M.</span> Very faint.</td>
            <td>2.30 <span class="smcapuc">A.M.</span></td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">22</td>
            <td>2 <span class="smcapuc">A.M.</span> Character not recorded.</td>
            <td>Very clear sky.</td>
          </tr>
          <tr>
            <td class="center bl nobr">”</td>
            <td class="center nobr">”</td>
            <td class="tdr">24</td>
            <td>Bright sky. Very faint.</td>
            <td>Midnight.</td>
          </tr>
          <tr>
            <td class="center nobr bl bb">”</td>
            <td class="center nobr bb">”</td>
            <td class="tdr bb">26</td>
            <td class="bb">10 <span class="smcapuc">P.M.</span> and 11 <span class="smcapuc">P.M.</span> Faint flashes.</td>
            <td class="bb">Sky obscured.</td>
          </tr>
        </table>

        <p>I have added to the above Table the character of the Aurora in each instance as taken from the fuller
        descriptions given.—J. R. C.</p>

        <h4><i>Auroras and Magnetic Disturbances.</i></h4>

        <p>The appearances of Auroras and the synchronous movements of the declinometer-magnet
        were subjects of special observation during the stay of the
        ‘Alert’ and ‘Discovery’ at their winter-quarters. The Table on page 187
        gives the dates and hours when Auroras were visible. On all occasions they
        were observed to be faint, with none of those brilliant manifestations which
        are described by our own officers as seen at Point Barrow, and by the Austro-Hungarian
        Expedition at Franz-Josef Land, where the magnetical instruments
        were so sensibly disturbed.</p>

        <p>These phenomena were not observed either in the ‘Alert’ or the ‘Discovery,’
        especially no connexion between magnetical disturbances and the
        appearances of Auroras could be traced.</p>

        <p>This is quite in accordance with the remarks of previous observers within
        the region comprehended between the meridians of 60° and 90° west, and
        north of the parallel of 73° north. For example:—</p>

        <p>In the Phil. Trans. 1826, Part IV. p. 76, Capt. Parry and Lieut. Foster
        remark, in the discussion of their magnetical observations at Port Bowen:—“As
        far, however, as our own observations extended, we have reason to believe
        that on no occasion were the needles in the slightest degree affected by
        Aurora, meteors, or any other perceptible atmospheric phenomenon.”</p>

        <p>Again, in the Smithsonian Contributions, vol. x., 1858, Mr. A. Schott, in<span class="pagenum"><a name="Page_187" id="Page_187">[187]</a><br /><a name="Page_188" id="Page_188">[188]</a></span>
        his discussion of Dr. Kane’s observations at Van Rensselaer Harbour, in
        1854, remarks—“In conformity with the supposed periodicity of this phenomenon
        as recognized by Professor Olmstead, no brilliant and complete
        Auroras have been seen; with an exception of very few, they may all be
        placed in his fourth class, to which the most simple forms of appearances
        have been referred.” The following statement is given in the same page
        as a footnote:—“The processes have no apparent connexion with the magnetic
        dip, and in <i>no</i> case did the needle of our unifilar indicate disturbance.”</p>

        <p class="separator"></p>
        <p>The following description of the Aurora observed on 21st November, 1875,
        is given by Commander Markham and Lieut. Giffard, in their abstract of
        observations at Floeberg Beach:—</p>

        <p>“Between 10 and 11 <span class="smcapuc">P.M.</span> bright broad streamers of the Aurora appeared
        10° or 15° above the north horizon, stretching through the zenith, and terminating
        in an irregular curve about 25° above the south horizon, bearing
        S.S.W. During the Aurora’s greatest brilliancy the magnet was observed
        during five minutes to be undisturbed.”</p>

        <div class="smaller">

        <p>[<i>Note.</i>—I applied for a loan of the lithographic stones to enable me to give copies of the three
        diagrams of Auroræ referred to in the Arctic “Results;” but the Lords Commissioners of H.M.
        Treasury refused this, except on the terms of my paying one third of the original cost of production
        of such diagrams. I did not think it worth while to accept these conditions. Only one of the
        drawings has any special interest; and this is a “curtain” Aurora, similar to that figured on
        Plate II. of this work.—J. R. C.]</p>

        </div>

      </section>

      <!-- Appendix D_________________________________________________________ -->
      <section class="appendix" id="appendix-d">


        <h3 class="titleapp" id="app-d">
          The Aurora and Ozone
        </h3>
        <p class="shorter">
          The Aurora and Ozone
        </p>





        <p><span class="sidenote">Aurora and ozone. Dr. Allnatt’s notes and conclusions deduced therefrom.</span>While Part I. was in the press, Dr. Allnatt, formerly of Frant, and for many
        years the well-known meteorological contributor to ‘The Times’ newspaper,
        kindly placed at my disposal his large series of notes. Upon an examination
        of these we came to the following conclusions:—</p>

        <p>1. That Auroral periods are also periods of comparative abundance of
        ozone.</p>

        <p>2. That instances are by no means wanting in which an abnormal development
        of ozone appears to be coincident with the manifestation of an
        Aurora.</p>



        <p>In reference to the first point, it is found, as the result of an examination
        of Dr. Allnatt’s notes, that particular years and months are notable at once
        for Auroræ and for ozone in abundance. <span class="sidenote">Year 1870 remarkable for sun-spots, auroræ, and ozone. <br/>Particulars of some of the monthly records.</span>1870 was one of these years, and
        was specially recorded by Dr. Allnatt, in his ‘Summary for the Year,’ as
        remarkable for sun-spots, Auroræ, and ozone.</p>


        <p>The month of February in that year was marked by intense cold and
        brilliant Auroræ. Atmospheric electricity was feeble, but ozone was,
        throughout the month, well developed; and there was no tangible period of
        antozone.</p>

        <p>In the month of April of the same year, eight days consecutively (19th to
        26th) were marked for ozone 10, the maximum of Dr. Allnatt’s scale.</p>

        <p>In May of the same year there were magnificent Auroræ, and atmospheric
        electricity was intense. Ozone was scanty; but this was accounted for by
        the wind being generally E.N.E., ozone being mostly developed with a W.
        or S.W. wind, and a moist state of the atmosphere.</p>


        <p>In August 1870 the unusually large number of 22 days were recorded for
        a maximum of ozone.</p>

        <p>September 1870 was hardly less remarkable, with 19 days of maximum.
        It was recorded that there were splendid Auroræ during this month, and the
        solar spots were very large.</p>

        <p>October 1870 had 20 days of maximum ozone, and November had several
        fine Auroræ and maxima of ozone noted. In fact, nearly every month in that
        year was referred to by Dr. Allnatt for displays of Aurora (of both Arctic and
        Antarctic forms) and for a development of ozone very considerably above the
        average.</p>



        <p><span class="sidenote">Year 1871.</span>The year 1871 had more or less of the same character. In the month of
        October of that year, fine Auroræ were prevalent, and ozone was registered as
        at its maximum during 22 days.</p>

        <p>There seems reason to conclude that if a systematic comparison of annual
        or other periods of Aurora and ozone development were made, it would result
        in disclosing a connexion (probably an intimate one) between the two phenomena.</p>



        <p><span class="sidenote">Instances showing a connexion between a specific Aurora and an ozone maximum.</span>With reference to the second point, the following (among other) instances
        may be quoted, for the purpose of showing a connexion between a specific
        Aurora and an ozone maximum.</p>

        <p>The Aurora of 24th September, 1870, was splendid and universal, being
        seen in Europe, Asia, Africa, America, and Australia. Ozone reached, on the
        morning of the 24th, 8 of the scale (the scale running from 1 to 10), and, on
        the morning of the 25th, 10, the maximum.</p>

        <p>In October 1870 there were grand displays on the 14th, 20th, 22nd, 24th,
        and 25th, and ozone was correspondingly abundant, as is seen by the following
        Table:—</p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Aurora.</th>
              <th>Ozone.</th>
              <th></th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1870,</td>
            <td class="nobr">October</td>
            <td>14th.</td>
            <td>Aurora.</td>
            <td class="tdr">8</td>
            <td rowspan="7" class="bb">The display of the 24th was accompanied by the formation of a
                corona, and that of the 25th was splendidly seen in Edinburgh.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>20th.</td>
            <td>Aurora.</td>
            <td class="tdr">10</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>21st.</td>
            <td class="nw">None seen.</td>
            <td class="tdr">5</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>22nd.</td>
            <td>Aurora.</td>
            <td class="tdr">10</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>23rd.</td>
            <td>None seen.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>24th.</td>
            <td>Aurora.</td>
            <td class="tdr">10</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">25th.</td>
            <td class="bb">Aurora.</td>
            <td class="tdr bb">8</td>
          </tr>
        </table>

        <p></p>

        <p>The foregoing figures somewhat point to the conclusion that ozone quantity
        rises and falls coincidently with the Aurora displays.</p>

        <p>The following seems, however, a case still more strongly in point.</p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Wind.</th>
              <th>Aurora.</th>
              <th>Ozone.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1871,</td>
            <td class="nobr">January</td>
            <td>25th.</td>
            <td>E.S.E.</td>
            <td>None seen.</td>
            <td class="tdr">0</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>26th.</td>
            <td>N.N.W.</td>
            <td>None seen.</td>
            <td class="tdr">2</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>27th.</td>
            <td>E.S.E.</td>
            <td>Aurora at night in N. and S. horizons.</td>
            <td class="tdr">10</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>28th.</td>
            <td>E.</td>
            <td>None seen.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">29th.</td>
            <td class="bb">S.E.</td>
            <td class="bb">None seen.</td>
            <td class="tdr bb">2</td>
          </tr>
        </table>

        <p>It is curious, in examining the above Table, to note how the ozone rose,
        notwithstanding an east wind, from 0 on the 25th, and 2 on the 26th, to 10
        on the 27th, when the Aurora appeared, and 8 on the 28th, when it might
        have lingered; and how it again descended to 2 on the 29th.</p>

        <p>The case of the Aurora of 6th of October, 1869, when a broad belt of
        Aurora was in the north, is also an illustrative one, as will be seen by the
        following data:—</p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Wind.</th>
              <th>Ozone.</th>
              <th>Aurora.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1869,</td>
            <td class="nobr">October</td>
            <td>5th.</td>
            <td>S.S.W.</td>
            <td class="tdr">1</td>
            <td>—</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>6th.</td>
            <td>S.S.E.</td>
            <td class="tdr">5</td>
            <td>Aurora.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>7th.</td>
            <td>S.S.W.</td>
            <td class="tdr">10</td>
            <td>—</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>8th.</td>
            <td>S.</td>
            <td class="tdr">10</td>
            <td>—</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">9th.</td>
            <td class="bb">S.E.</td>
            <td class="tdr bb">5</td>
            <td class="bb">—</td>
          </tr>
        </table>

        <p>The Aurora of the night of the 6th was here represented by the ozone-paper
        of the morning of the 7th with a maximum of 10, which lasted till
        the 8th.</p>

        <p>[It should be borne in mind, in examining these Tables, that the
        Aurora is of the night of the given date, while the ozone-papers are taken
        and recorded in the morning of the date quoted.]</p>



        <p><span class="sidenote">Other instances.</span>We will now take instances where the ozone has not reached its maximum;
        but even in these cases a certain amount of rise and fall of the ozone development
        towards and from the Aurora is traceable.</p>

        <p></p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Wind.</th>
              <th>Ozone.</th>
              <th>Aurora.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1871,</td>
            <td class="nobr">April</td>
            <td>8th</td>
            <td>S.S.E.</td>
            <td class="tdr">5</td>
            <td rowspan="3" class="bb">Aurora on 9th, but wind E. and unfavourable to ozone.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>9th</td>
            <td>S.S.E.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">10th</td>
            <td class="bb">S.E.</td>
            <td class="tdr bb">5</td>
          </tr>
          <tr>
            <td class="bl nobr">1871,</td>
            <td class="nobr">November</td>
            <td>9th</td>
            <td>N.</td>
            <td class="tdr">5</td>
            <td rowspan="3" class="bb">Aurora on all three nights.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>10th</td>
            <td>N.W.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">11th</td>
            <td class="bb">N.</td>
            <td class="tdr bb">5</td>
          </tr>
          <tr>
            <td class="bl nobr">1872,</td>
            <td class="nobr">February</td>
            <td>3rd</td>
            <td>S.W.</td>
            <td class="tdr">4</td>
            <td rowspan="4" class="bb">Aurora on night of the 4th represented by ozone-paper of morning of the 5th.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>4th</td>
            <td>S.S.W.</td>
            <td class="tdr">5</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>5th</td>
            <td>S.W.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">6th</td>
            <td class="bb">S.W.</td>
            <td class="tdr bb">5</td>
          </tr>
        </table>

        <p>Other cases are, we are bound to say, found, in which ozone was either not
        remarkable for quantity, or positively fell during the Aurora, as, for instance,
        this:—</p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Wind.</th>
              <th>Ozone.</th>
              <th>Aurora.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1874,</td>
            <td class="nobr">March</td>
            <td>16th</td>
            <td>W.N.W.</td>
            <td class="tdr">6</td>
            <td rowspan="4" class="bb">Aurora on the 18th represented by test-paper
            of the 19th with only two degrees of discoloration.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>17th</td>
            <td>S.W.</td>
            <td class="tdr">6</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>18th</td>
            <td>W.</td>
            <td class="tdr">5</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">19th</td>
            <td class="bb">S.S.W.</td>
            <td class="tdr bb">2</td>
          </tr>
        </table>

        <p>It is, however, possible that such instances may be accounted for, either
        by some reaction on the test-papers after they have been coloured, or by
        some accidental antagonistic circumstance affecting the tests. The following
        is a case well illustrating this:—</p>

        <table summary="" class="borders">
          <thead>
            <tr>
              <th colspan="3">Date.</th>
              <th>Wind.</th>
              <th>Ozone.</th>
              <th>Aurora.</th>
            </tr>
          </thead>
          <tr>
            <td class="bl nobr">1874,</td>
            <td class="nobr">January</td>
            <td>31st</td>
            <td>N.N.W.</td>
            <td class="tdr">6</td>
            <td rowspan="5" class="bb">There was an Aurora on the night of the 2nd represented by the ozone-paper (4 only) on the morning of the 3rd.</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr">February</td>
            <td>1st</td>
            <td>N.W.</td>
            <td class="tdr">8</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>2nd</td>
            <td>N.W.</td>
            <td class="tdr">2</td>
          </tr>
          <tr>
            <td class="bl nobr center">”</td>
            <td class="nobr center">”</td>
            <td>3rd</td>
            <td>N.N.W.</td>
            <td class="tdr">4</td>
          </tr>
          <tr>
            <td class="bl nobr center bb">”</td>
            <td class="nobr center bb">”</td>
            <td class="bb">4th</td>
            <td class="bb">E.N.E.</td>
            <td class="tdr bb">8</td>
          </tr>
        </table>

        <p>This instance would seem strongly opposed to the theory of a connexion
        between Aurora and ozone but for the fact that on the 2nd, when the Aurora
        was seen at night, and on other days in the same month, Dr. Allnatt has
        recorded a strong wave of antozone to have swept over the whole of England,
        and blanched the ozone-papers, however deep their coloration might have
        previously been. Indeed, it is easy to understand that some antozonic
        influence may, at times, disturb the evidence of the test-papers, even in so
        elevated and apparently pure an atmosphere as that of Frant.</p>

        <p>It may not be considered that the foregoing instances are enough to establish
        a case of ozone=Aurora; but there seems, at least, sufficient to base a
        requisition for further inquiry upon.</p>

        <p>It would, too, be interesting to investigate whether Auroræ and ozone
        development are respectively localized. Mr. Ingall’s fine Aurora, seen at
        Champion Hill, S.E., July 18th, 1874 (<i>antè</i>, pp. 22 and 23), was not observed
        at Frant, and the ozonoscopes there were described as blanched by antozone.</p>

        </section>

        <!-- Appendix E_________________________________________________________ -->
      <section class="appendix" id="appendix-e">

        <h3 class="titleapp" id="app-e">
          Inquiries into the spectrum of aurora
        </h3>
        <p class="shorter">
          Inquiries into the spectrum of aurora
        </p>


          <p class="note-titre"><span class="smcap">By H. C. Vogel.</span>&nbsp;<span class="footnote">Communicated by the author to the Royal Saxon Academy of Science, 1871.</span></p>

          <p>The frequent appearance of the Aurora in the past winter, as well as this
          spring, has given me opportunity to institute exact inquiries into the spectrum
          of the Aurora. It is known that the nature of Auroræ is as yet but little
          explored. It has been considered necessary to abandon the former view—that
          they are discharges of the electricity collected at the poles—because it
          has been hitherto found impossible to bring the chief lines of the Aurora-spectrum
          into coincidence with the spectra of the atmospheric gases. Theoretical
          considerations, based on the great alterations to which the spectrum
          of the same gas is subject under varying conditions of temperature and
          density, have very recently led Zöllner to the opinion that probably the
          spectrum of the <i>Aurora does not coincide with any known spectrum</i> of
          the atmospheric gases, only because it is a spectrum of another form of our
          atmosphere hitherto incapable of artificial demonstration&nbsp;<span class="footnote">Reports of the Royal Saxon Academy of Science, Oct. 31, 1871.</span>.</p>

          <p>The following article will show how far I have succeeded, in conjunction
          with Dr. Lohse, in supporting this view by exact observations of the Aurora-spectrum
          itself, as well as by comparison with the spectra of the gases
          constituting the air.</p>

          <p>The star-spectrum apparatus belonging to the 11-inch equatorial of the
          Bothkamp Observatory was used for these observations. It consists of a
          set of prisms <i>à vision directe</i>, five prisms with refracting angle 90°, slit,
          collimator, and observing telescope. The lowest eyepiece (magnifying four
          times) of this telescope was employed. The telescope is capable of being
          moved in such a way, by the aid of a micrometer-screw, that different
          portions of the spectrum can be brought into the centre of the field of
          vision. As fractions of the rotation of this screw are marked, the distances
          of the spectral lines can be readily found.</p>

          <p>Repeated measurements of 100 lines of the solar spectrum have enabled
          me, upon the basis of Ångström’s Atlas (‘Spectre normal de Soleil’), to express
          the indications of the screw directly in wave-lengths.</p>

          <p>In place of the cross wires originally introduced into the focus of the
          observing telescope, I have inserted a tiny polished steel cone, the very fine
          point of which reaches to the centre of the field of vision. The axis of this
          cone stands perpendicular to the length of the spectrum, therefore parallel
          with the spectral lines, and the setting of the point of the cone on the latter
          is accomplished with great sharpness. If the spectrum is very faint, or
          consists only of bright lines, the cone is lighted by a small lamp. For this
          purpose, opposite to the point of the cone, there is an opening in the
          telescope, through which, regulated by a blind, light can be thrown on the
          point. As the latter is polished, a fine line of light thus appears, which
          extends to the centre of the field of vision, and the brilliancy of which can
          be altered by withdrawing the lamp to a greater distance or lowering the
          blind, so that even the faintest lines of a spectrum can be brought with
          facility and certainty into coincidence with this line of light.</p>

          <p>The head of the micrometer-screw is divided into 100 parts, and each part,
          in the neighbourhood of the Fraunhofer line F, answers to about ·00016
          wave-length. The probable error of position on one of the well-marked lines
          in the sun’s spectrum amounts to about 0·008 of a turn of the screw with the
          lowest eyepiece of the telescope. I have subjected the screw itself to a
          thorough examination with reference to such range, as well as to periodical
          inequalities in the single worms of the screw, but could discover no error
          exceeding 0·01 of a turn of the screw. I have to mention, further, that after
          each observation in the position in which the instrument was used, readings
          followed on the sodium-lines, or on some of the hydrogen-lines, in order to
          eliminate errors which might arise in the unavoidable disturbance of any
          particular part of the spectral apparatus.</p>

          <h4 class="h4-appe">Observations of the Aurora</h4>

          <p>1870, Oct. 25th.—A very bright Aurora. In the brightest parts, besides
          a very bright line between D and E, several other fainter lines were to be
          discerned, situated further towards the blue end of the spectrum. They
          appeared on a dimly-lighted ground, and stretched out over the Fraunhofer
          lines E and <i>b</i> to about midway between <i>b</i> and F. Towards the red end the
          spectrum was terminated by the bright line first mentioned. No measurements
          could be taken, as the apparatus had not yet undergone the above-mentioned
          alterations, and even the brightest line of the spectrum did not
          diffuse sufficient light to be able to perceive the fine cross wires. The red
          rays of the Aurora were not examined.</p>

          <p>1871, Feb. 11th.—Towards ten o’clock appeared in the north-west a very
          bright light-bow of greenish colour as the edge of a dark segment. Even
          with a very narrow slit, the line between D and E could be well recognized
          and measured. The average of six readings gave 7·11 turns, equal to 5572
          wave-length. In a small spectroscope of low dispersion which is arranged
          on Browning’s plan, a few more lines placed further towards the blue could
          be recognized (as in October). Towards the red end of the spectrum no lines
          were observable. The greatest development of the Aurora was about
          midnight. Magnificent rays rose to about 60° elevation; they had the
          same greenish colouring as the bow of light, and the appearance of the
          spectrum also was exactly the same. I again obtained two sets of measurements:
          the average of six readings in the first set gave 7·10 turns, 5572
          wave-length; in another part of the heavens at the same time 7·10 was the
          result of four readings.</p>

          <p>On Feb. 12, towards eight o’clock, the intensity of the Aurora was already
          great enough to allow measurements of the brightest line. The average of
          six readings gave 7·09 turns, or 5576 wave-length. Dr. Lohse took observations
          later, with the same apparatus, and found from six readings 7·12 turns,
          or 5569 wave-length.</p>

          <p>Yet the appearance of the spectrum in the spectroscope of low dispersion
          was essentially distinct from that of February 11th. The green continuous
          spectrum was present; it extended from the bright Aurora-line to the lines
          <i>b</i> of the solar spectrum, and was traversed by some bright lines. Between
          band <i>b</i> and F, was another line standing alone, out beyond F, in the blue part
          of the spectrum, a clear bright stripe; and just before G a very faint broad
          band of light was perceived.</p>

          <p>Amongst the rays which, later on, shot upwards, and were coloured red at
          their ends, another very intense red line appeared in the spectrum between
          C and D, yet placed nearer to C&nbsp;<span class="footnote">This red line was first noticed by Zöllner.</span>.</p>

          <p>April 9th.—An exceedingly brilliant Aurora, of which the greater development
          took place in the early morning hours. Magnificent red sheaths rose
          up to the zenith. The spectrum was like that observed on February 12th,
          only much more intense, so that the lines could be seen and measured with
          the larger spectral apparatus. In the brightest part of the Aurora was
          the dark segment; the spectrum consisted of five lines in the green, and a
          somewhat indistinct broad line or band in the blue.</p>

          <p>The red rays, on the other hand, allowed us to recognize seven lines, whilst
          the bright line again appeared in the red part of the spectrum. I could not
          again perceive the faint stripe observed on February 12th, in the vicinity of
          line G. The mean measurements of four readings on an average, for each
          line, gave:—</p>

          <table summary="" class="borders">
            <thead>
              <tr>
                <th>Turns of<br />screw.</th>
                <th>Probable<br />errors.</th>
                <th>Wave-length.</th>
                <th>Probable<br />errors.</th>
                <th colspan="2">Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">4·62</td>
              <td class="tdr">·0037</td>
              <td class="tdr">6297</td>
              <td class="tdr">·00014</td>
              <td>Very bright stripe.</td>
              <td rowspan="3">On a faintly lighted ground.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·12</td>
              <td class="tdr">9</td>
              <td class="tdr">5569</td>
              <td class="tdr">2</td>
              <td>Brightest line of the spectrum; becomes noticeably fainter at appearance of the red line.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·92</td>
              <td class="tdr">—</td>
              <td class="tdr">5390</td>
              <td class="tdr">—</td>
              <td>Extremely faint line; unreliable observation.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·71</td>
              <td class="tdr">21</td>
              <td class="tdr">5233</td>
              <td class="tdr">4</td>
              <td colspan="2">Moderately bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·95</td>
              <td class="tdr">49</td>
              <td class="tdr">5189</td>
              <td class="tdr">9</td>
              <td colspan="2">This line is very bright when the red line appears at the same time, otherwise equal in brilliancy with the preceding one.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·06</td>
              <td class="tdr">20</td>
              <td class="tdr">5004</td>
              <td class="tdr">3</td>
              <td colspan="2">Very bright line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">12·33</td>
              <td class="tdr">—</td>
              <td class="tdr">4694</td>
              <td class="tdr">—</td>
              <td colspan="2" rowspan="2">Broad band of light, somewhat less brilliant in the middle.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·59</td>
              <td class="tdr">22</td>
              <td class="tdr">4663</td>
              <td class="tdr">—</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr bb">12·88</td>
              <td class="bb">—</td>
              <td class="tdr bb">4629</td>
              <td class="tdr bb">3</td>
              <td class="bb" colspan="2">Very faint in those parts of the Aurora in which the red line appears.</td>
            </tr>
          </table>

          <p>April 14th.—Faint Aurora; only the bright line in the green could
          be recognized in its spectrum. The mean of two readings gave 7·12 turns, or
          5569 wave-length.</p>

          <p>I append a table of the wave-lengths of the brightest line, as exactly
          measured on four evenings:—</p>

          <table summary="Wave-lengths observed on four different dates" class="table-noborder">
            <thead>
              <tr>
                <td>1871,</td>
                <td>February</td>
                <td class="tdr">11</td>
                <td class="tdr">5573</td>
              </tr>
            </thead>
            <tr>
              <td></td>
              <td class="center">”</td>
              <td class="tdr">12</td>
              <td class="tdr">5573</td>
            </tr>
            <tr>
              <td></td>
              <td>April</td>
              <td class="tdr">9</td>
              <td class="tdr">5569</td>
            </tr>
            <tr>
              <td></td>
              <td class="center">”</td>
              <td class="tdr">14</td>
              <td class="tdr">5569</td>
            </tr>
          </table>

          <p>Therefore the average result (if only half-weight is allowed to the last
          observation, because it only depends upon two readings) gives for the wave-length
          of the brightest line 5571·3, with a probable error of ·000·92.
          According to Ångström&nbsp;<span class="footnote">Recherches sur le Spectre Solaire, p. 42.</span>, the wave-length of this line is 5567; according to
          Winlock&nbsp;<span class="footnote">American Journal of Science, lxviii. 123.</span>, on the other hand, 5570.</p>

          <h4 class="h4-appe">On the Spectra of some Gases in Geissler’s Tubes, as well as on the
          Spectrum of the Atmospheric Air.</h4>

          <p>Numerous experiments have been made in order to find out some admitted
          connexion between the spectrum of the Aurora and the spectra of the
          principal gases composing the atmosphere. I limit myself to noticing some
          of the often-repeated observations in Plücker’s tubes, which contained oxygen,
          hydrogen, and nitrogen, as well as the observations of the spectrum of the
          air under different conditions. The experiments were made with a small
          inductive apparatus, in which the length of the spark between platinum
          points in ordinary air was 15 millims. at the most. As Zöllner (in the
          pamphlet mentioned) comes to the conclusion, that if the development of the
          light in the Aurora, according to the analogy of gases brought to glow in
          rarefied spaces, is of an electric nature, it must belong to very low temperature—in
          order to bring the gases enclosed in the tubes to glow at the
          lowest possible temperature, I have always employed such weak currents that
          the gas was only just steadily alight.</p>

          <p>The following observations have been repeated often and at various times.
          The figures are averages of the indications of the micrometer-screw, so that
          the uncertainty of the figures will, in the rarest cases, amount to no more than
          0·015 turn of the screw, and must be reckoned somewhat more highly only
          in the case of completely faint misty lines. The spectrum apparatus was that
          described above, and the slit was nearly the same in every experiment, and so
          narrow that the sodium-lines could be seen separated. The measurements,
          for the most part, extend only to the Fraunhofer line G, as I feared lest,
          through further turning the telescope by means of the micrometer-screw, too
          great a pressure might be exercised on the worms of the latter.</p>

          <p></p>

          <h5>Oxygen</h5>



          <table summary="" class="borders">
            <caption>In the narrow part of the Plücker tube</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">3·97</td>
              <td class="tdr">6562</td>
              <td>Moderately bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">5·04</td>
              <td class="tdr">6146</td>
              <td>Very bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">6·98</td>
              <td class="tdr">5603</td>
              <td>Very bright, misty towards the violet.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·19</td>
              <td class="tdr">5332</td>
              <td>Faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·95</td>
              <td class="tdr">5189</td>
              <td>Moderately bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·97</td>
              <td class="tdr">4870</td>
              <td><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">11·02</td>
              <td class="tdr">4863</td>
              <td>Faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">11·26</td>
              <td class="tdr">4829</td>
              <td>Bright; misty towards the red end of the spectrum.</td>
            </tr>
            <tr>
              <td class="bl tdr">13·30</td>
              <td class="tdr">4583</td>
              <td>Very faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">14·05</td>
              <td class="tdr">4506</td>
              <td>Moderately bright.</td>
            </tr>
            <tr>
              <td class="bl tdr bb">15·55</td>
              <td class="tdr bb">4372</td>
              <td class="bb"><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
          </table>



          <table summary="" class="borders">
            <caption>In the wide part of the Plücker tube</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">6·98</td>
              <td class="tdr">5603</td>
              <td>Very faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·95</td>
              <td class="tdr">5189</td>
              <td>Very bright.</td>
            </tr>
            <tr>
              <td class="bl tdr bb">11·26</td>
              <td class="tdr bb">4829</td>
              <td class="bb">Moderately bright.</td>
            </tr>
          </table>

          <p>The lines near 3·97 and 11·02 belong to hydrogen. Probably traces of
          aqueous vapour were present in the tube, which were decomposed by the
          galvanic current. These two lines are not to be found in a lower temperature
          in the broad part of the tube. It is striking that the red nitrogen-line near
          5·04 is also missing there. In the narrow part of the tube the lines stand out
          in the green on a very dimly-lighted ground, whilst in the wider part they
          appear on a perfectly dark ground.</p>


          <h5>Hydrogen.</h5>



          <table summary="" class="borders">
            <caption> In the narrow part of the tube.</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th colspan="2">Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">3·98</td>
              <td class="tdr">6558</td>
              <td colspan="2">Very bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">6·16</td>
              <td class="tdr">5813</td>
              <td colspan="2">Moderately bright, on both sides very faint lines.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">7·01</td>
              <td class="tdr">5596</td>
              <td class="nw nobr">Moderately bright.</td>
              <td rowspan="3" class="hanging"><span class="x2">}</span>On a dimly lighted ground, which becomes fainter towards the violet.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·18</td>
              <td class="tdr">5555</td>
              <td class="nobr">Moderately bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·77</td>
              <td class="tdr">5422</td>
              <td class="nobr">Faint.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">8·95</td>
              <td class="tdr">5189</td>
              <td class="nobr">Moderately bright.</td>
              <td rowspan="3" class="hanging"><span class="x2">}</span>On a faint steadily bright ground.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·03</td>
              <td class="tdr">5008</td>
              <td class="nobr">Faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·55</td>
              <td class="tdr">4929</td>
              <td class="nobr">Moderately bright.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">11·04</td>
              <td class="tdr">4861</td>
              <td class="nobr">Very bright.</td>
              <td rowspan="2" class="hanging"><span class="x2">}</span>From 11·5 to 12·9 a bright ground, which towards the violet becomes very bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·86</td>
              <td class="tdr">4632</td>
              <td class="nobr">Moderately bright.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">13·32</td>
              <td class="tdr">4581</td>
              <td class="nobr">Very faint.</td>
              <td rowspan="3" class="bb hanging"><span class="x2">}</span>&nbsp;On a dull ground.</td>
            </tr>
            <tr>
              <td class="bl tdr">14·05</td>
              <td class="tdr">4506</td>
              <td class="nobr"><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr bb">15·90</td>
              <td class="tdr bb">4342</td>
              <td class="nobr bb">Very bright.</td>
            </tr>
          </table>



          <table summary="" class="borders">
            <caption> In the broad part of the tube.</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">5·30</td>
              <td class="tdr">6063</td>
              <td>Faint.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·00</td>
              <td class="tdr">5598</td>
              <td>Bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·96</td>
              <td class="tdr">5187</td>
              <td>Very bright.</td>
            </tr>
            <tr>
              <td class="bl tdr">11·28</td>
              <td class="tdr">4828</td>
              <td><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr bb">14·04</td>
              <td class="tdr bb">4507</td>
              <td class="bb">Moderately bright.</td>
            </tr>
          </table>

          <p>The lines appeared on a perfectly dark ground.</p>

          <p>The tube shows in the narrow part the hydrogen-spectrum of the first order;
          the lines in the green do not coincide with the lines of the nitrogen, though
          some lines belonging to nitrogen are found. Here, too, most probably small
          particles of aqueous vapour have been enclosed in the tube and are decomposed.
          Very striking is the spectrum in the broad part of the tube; nothing
          is to be seen of the bright shining lines Hα 3·98, Hβ 11·04, Hγ 15·90; on
          the other hand, four very bright lines and one quite faint one are in the red
          end of the spectrum, which appear, in opposition to the spectrum of the
          narrow part, not on a partially lighted, but on an entirely dark ground. The
          appearance is very striking if we bring the tube in front of the slit; and so,
          by degrees, at first the light in the narrow part, then the light at the connecting-point
          of the narrow and wide parts, and, finally, the light in the
          latter fall upon the slit. At the connecting-point of the wide ends of the
          tube the three well-known hydrogen-lines decrease in intensity, the continuous
          ground of some parts of the spectrum disappears, and a new line
          appears near 11·28, which has about the same brilliancy as Hβ.</p>

          <p>A comparison with the spectrum of oxygen shows the bright lines which
          are in the spectrum in the wide end of the tube as belonging to that element.
          The heat evolved by the current appears insufficient to bring the hydrogen to
          glow, whilst by it the oxygen, which is of a more rarefied character, becomes
          incandescent. An alteration of the direction of the current has no influence
          on the appearance.</p>

          <h5>Nitrogen</h5>



          <table summary="" class="borders">
            <caption> In the narrow part of the tube</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th colspan="2">Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">3·84</td>
              <td class="tdr">6620</td>
              <td rowspan="2" colspan="2">Several faint, broad, close lines, increasing in brilliancy as they approach the violet end.</td>
            </tr>
            <tr>
              <td class="bl tdr">4·85</td>
              <td class="tdr">6213</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">5·30</td>
              <td class="tdr">6063</td>
              <td rowspan="7" colspan="2">Broad bright lines, so close together that the intervening spaces appear like fine dark lines. This part of the spectrum is very bright, but not uniform, being brighter towards the violet end.</td>
            </tr>
            <tr>
              <td class="bl tdr">5·51</td>
              <td class="tdr">6000</td>
            </tr>
            <tr>
              <td class="bl tdr">5·69</td>
              <td class="tdr">5948</td>
            </tr>
            <tr>
              <td class="bl tdr">5·87</td>
              <td class="tdr">5896</td>
            </tr>
            <tr>
              <td class="bl tdr">6·04</td>
              <td class="tdr">5846</td>
            </tr>
            <tr>
              <td class="bl tdr">6·20</td>
              <td class="tdr">5802</td>
            </tr>
            <tr>
              <td class="bl tdr">6·43</td>
              <td class="tdr">5741</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">6·96</td>
              <td class="tdr">5607</td>
              <td rowspan="3" colspan="2">Group of faint but at the same time very broad lines. The last is the brightest.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·13</td>
              <td class="tdr">5567</td>
            </tr>
            <tr>
              <td class="bl tdr">7·28</td>
              <td class="tdr">5532</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">7·55</td>
              <td class="tdr">5470</td>
              <td rowspan="4" colspan="2">The dark intervening spaces are somewhat broader, the bright lines
                  somewhat more intense than in the preceding group, and all of
                  almost equal brilliancy.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·74</td>
              <td class="tdr">5428</td>
            </tr>
            <tr>
              <td class="bl tdr">7·92</td>
              <td class="tdr">5389</td>
            </tr>
            <tr>
              <td class="bl tdr">8·09</td>
              <td class="tdr">5353</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">8·32</td>
              <td class="tdr">5306</td>
              <td colspan="2" rowspan="3">Very faint fine lines.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·50</td>
              <td class="tdr">5272</td>
            </tr>
            <tr>
              <td class="bl tdr">8·69</td>
              <td class="tdr">5237</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">9·01</td>
              <td class="tdr">5178</td>
              <td colspan="2">Very bright broad misty line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">9·67</td>
              <td class="tdr">5066</td>
              <td>Very bright line.</td>
              <td rowspan="11">The bright lines are sharply defined towards the red end of the
                  spectrum, fading away towards the other end of the spectrum.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·25</td>
              <td class="tdr">4975</td>
              <td><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">10·66</td>
              <td class="tdr">4913</td>
              <td><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">11·03</td>
              <td class="tdr">4862</td>
              <td>Very faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">11·41</td>
              <td class="tdr">4811</td>
              <td>Bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·11</td>
              <td class="tdr">4721</td>
              <td><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">12·57</td>
              <td class="tdr">4666</td>
              <td>Faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·57</td>
              <td class="tdr">4644</td>
              <td class="nw">Bright, broad, misty line.</td>
            </tr>
            <tr>
              <td class="bl tdr">13·42</td>
              <td class="tdr">4570</td>
              <td>Very bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">14·24</td>
              <td class="tdr">4487</td>
              <td><span class="ditto2">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">15·02</td>
              <td class="tdr">4417</td>
              <td>Bright line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">15·66</td>
              <td class="tdr">4363</td>
              <td class="nobr">Bright lines.</td>
              <td rowspan="4" class="bb">Bright lines sharply defined towards red end, indistinct towards other end of spectrum.</td>
            </tr>
            <tr>
              <td class="bl tdr">15·72</td>
              <td class="tdr">4357</td>
              <td class="nobr"><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">15·87</td>
              <td class="tdr">4345</td>
              <td class="nobr">Bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr bb">16·72</td>
              <td class="tdr bb">4273</td>
              <td class="nobr bb"><span class="ditto2">”</span></td>
            </tr>
          </table>

          <p class="center">Here follow several lines.</p>




          <table summary="" class="borders">
            <caption> In the wide part of the tube</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">6·20</td>
              <td class="tdr">5802</td>
              <td>Faint, indistinct, broad line.</td>
            </tr>
            <tr>
              <td class="bl tdr">7·72</td>
              <td class="tdr">5433</td>
              <td>Dull stripe.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·20</td>
              <td class="tdr">5330</td>
              <td>Faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·94</td>
              <td class="tdr">5191</td>
              <td>Very faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">9·03</td>
              <td class="tdr">5175</td>
              <td>Broad band of light.</td>
            </tr>
            <tr>
              <td class="bl tdr">9·90</td>
              <td class="tdr">5029</td>
              <td>Dull band of light.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·68</td>
              <td class="tdr">4911</td>
              <td>Moderately bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">11·42</td>
              <td class="tdr">4809</td>
              <td>Faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·59</td>
              <td class="tdr">4663</td>
              <td>Bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">13·43</td>
              <td class="tdr">4569</td>
              <td><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr">14·07</td>
              <td class="tdr">4504</td>
              <td>Moderately bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">14·25</td>
              <td class="tdr">4486</td>
              <td>Very bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr">15·85</td>
              <td class="tdr">4347</td>
              <td><span class="ditto1">”</span><span class="ditto1">”</span></td>
            </tr>
            <tr>
              <td class="bl tdr bb">16·76</td>
              <td class="tdr bb">4273</td>
              <td class="bb">Moderately bright line.</td>
            </tr>
          </table>



          <table summary="" class="borders">
            <caption>At the aura of the negative pole</caption>
            <thead>
              <tr>
                <th>Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl tdr">5·18</td>
              <td class="tdr">6100</td>
              <td rowspan="2">Broad, moderately bright stripe, indistinct at the edges.</td>
            </tr>
            <tr>
              <td class="bl tdr">5·70</td>
              <td class="tdr">5945</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">7·60</td>
              <td class="tdr">5159</td>
              <td rowspan="2">Broad, moderately bright stripe.</td>
            </tr>
            <tr>
              <td class="bl tdr">8·41</td>
              <td class="tdr">5289</td>
            </tr>
            <tr class="spaced">
              <td class="bl tdr">8·76</td>
              <td class="tdr">5224</td>
              <td>Very bright line, somewhat indistinct towards the violet.</td>
            </tr>
            <tr>
              <td class="bl tdr">9·19</td>
              <td class="tdr">5147</td>
              <td>Faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·00</td>
              <td class="tdr">5004</td>
              <td>Bright line, indistinct towards the red.</td>
            </tr>
            <tr>
              <td class="bl tdr">10·67</td>
              <td class="tdr">4912</td>
              <td>Somewhat fainter than the last, indistinct towards the red.</td>
            </tr>
            <tr>
              <td class="bl tdr">11·43</td>
              <td class="tdr">4808</td>
              <td>Very faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·25</td>
              <td class="tdr">4704</td>
              <td>Very intense, broad, indistinct towards the violet.</td>
            </tr>
            <tr>
              <td class="bl tdr">12·73</td>
              <td class="tdr">4646</td>
              <td>Very faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">13·43</td>
              <td class="tdr">4569</td>
              <td>Moderately bright, indistinct towards the violet.</td>
            </tr>
            <tr>
              <td class="bl tdr">14·25</td>
              <td class="tdr">4486</td>
              <td>Like the last.</td>
            </tr>
            <tr>
              <td class="bl tdr">15·03</td>
              <td class="tdr">4417</td>
              <td>Quite a faint line.</td>
            </tr>
            <tr>
              <td class="bl tdr">15·86</td>
              <td class="tdr">4346</td>
              <td>Moderately bright line.</td>
            </tr>
            <tr>
              <td class="bl tdr bb">16·76</td>
              <td class="tdr bb">4275</td>
              <td class="bb">Very bright line.</td>
            </tr>
          </table>

          <p class="center">Here follow several other lines.</p>


          <p>The observations in the different parts of the tube show plainly the
          dependence of the spectrum on the temperature. The aura of the negative
          pole gives the line near 10·07 so characteristic of the air-spectrum. This is
          the same line which is met with in the spectra of most of the nebulæ. The
          very striking groups of lines in the red and yellow in the spectrum of the
          narrow part of the tube disappear entirely in the wide part. If we compare
          the spectra with those above quoted, of oxygen and hydrogen, we find line
          Hβ very faint in the spectrum of the narrow part of the tube near 11·03;
          on the other hand, oxygen-lines appear in the broad part near 8·20, 8·94, and
          14·07. Thence I would conjecture that the tube was not filled with pure
          nitrogen, the appearance of which is precise, but with dry rarefied air, since
          Wüllner’s researches have proved that dry air yields the same spectrum as
          nitrogen gas. Perhaps the air in the tube examined by me had not been
          thoroughly dried, and thus the appearance of some lines of the elements
          before named is to be explained.</p>

          <p>I must further mention that the electrodes of the tubes consisted of
          aluminium; yet a comparison of the spectra observed and the aluminium
          spectrum has shown no connexion between them.</p>

          <h5>Atmospheric Air</h5>

          <table summary="" class="borders">
            <caption>Atmospheric Air</caption>
            <thead>
              <tr>
                <th colspan="2">Screw.</th>
                <th>Wave-length.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">5·88</td>
              <td class="tdr">5892</td>
              <td>Very bright double line (Na).</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">6·67</td>
              <td class="tdr">5680</td>
              <td>Very bright line.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">7·20</td>
              <td class="tdr">5550</td>
              <td>Faint line.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">9·00</td>
              <td class="tdr">5180</td>
              <td>Very bright line.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">9·79</td>
              <td class="tdr">5047</td>
              <td>Fine faint line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">10·03</td>
              <td class="tdr">5008</td>
              <td rowspan="2">Very bright double line.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">10·07</td>
              <td class="tdr">5002</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">11·43</td>
              <td class="tdr">4803</td>
              <td>Faint confused line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">12·69</td>
              <td class="tdr">4651</td>
              <td rowspan="3">Faint line not sharply defined.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">12·84</td>
              <td class="tdr">4633</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">13·04</td>
              <td class="tdr">4612</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr">From</td>
              <td class="tdr">14·61</td>
              <td class="tdr">4453</td>
              <td rowspan="2" class="bb">Confused band of light, which ends with a broad washy line.</td>
            </tr>
            <tr>
              <td class="bl nobr bb">to</td>
              <td class="tdr bb">15·88</td>
              <td class="tdr bb">4444</td>
            </tr>
          </table>

          <p class="center">Here follow several other lines.</p>




          <table summary="" class="borders">
            <caption>Rarefied air saturated with aqueous vapour.</caption>
            <thead>
              <tr>
                <th colspan="2">Screw.</th>
                <th>Wave-length.</th>
                <th colspan="2">Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">3·97&nbsp;</td>
              <td class="tdr">6562</td>
              <td colspan="2">Moderately bright line.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">(5·88)</td>
              <td class="tdr"> 5892</td>
              <td colspan="2">Bright double line (Na).</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">(6·25)</td>
              <td class="tdr">5789</td>
              <td colspan="2">Bright line (H).</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr">From</td>
              <td class="tdr">7·03&nbsp;</td>
              <td class="tdr">5591</td>
              <td colspan="2" rowspan="2">Broad dull band of light; near 7·03 a somewhat brighter line.</td>
            </tr>
            <tr>
              <td class="bl nobr">to</td>
              <td class="tdr">7·55&nbsp;</td>
              <td class="tdr">5470</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">7·59&nbsp;</td>
              <td class="tdr">5461</td>
              <td colspan="2">Bright line (H).</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">8·72&nbsp;</td>
              <td class="tdr">5231</td>
              <td colspan="2">Dull stripe.</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">8·96&nbsp;</td>
              <td class="tdr">5187</td>
              <td>Broad misty stripe.</td>
              <td rowspan="2">On a dull steady ground.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">10·07&nbsp;</td>
              <td class="tdr">5002</td>
              <td>Faint line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">11·05&nbsp;</td>
              <td class="tdr">4859</td>
              <td colspan="2">Very bright line.</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">12·21&nbsp;</td>
              <td class="tdr">4709</td>
              <td>Moderately bright line.</td>
              <td rowspan="3">On dimly lighted ground, becoming fainter towards the violet.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">12·75&nbsp;</td>
              <td class="tdr">4644</td>
              <td class="nw">Line fainter than the preceding.</td>
            </tr>
            <tr>
              <td class="bl nobr"></td>
              <td class="tdr">13·28&nbsp;</td>
              <td class="tdr">5585</td>
              <td>Very faint line (H).</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr"></td>
              <td class="tdr">(15·71)</td>
              <td class="tdr">4358</td>
              <td colspan="2">Very bright line (H).</td>
            </tr>
            <tr>
              <td class="bl nobr bb"></td>
              <td class="tdr bb">15·90&nbsp;</td>
              <td class="tdr bb">4341</td>
              <td colspan="2" class="bb"><span class="ditto1">”</span> <span class="ditto1">”</span></td>
            </tr>
          </table>

          <p class="center">Here follow several more lines.</p>

          <p>In the first observations, the electric spark, about 1 centim. in length, was
          allowed to pass between platinum points in ordinary air.</p>

          <p>The sodium-line near 5·88 appeared continually. The bright double line
          at 10·03 and 10·07, with a weaker current or longer spark, was no longer to
          be recognized as a double line, but appeared as a broad somewhat confused
          line, of which the brightest part was near 10·05. No lines belonging to the
          platinum spectrum appeared. Ordinary rarefied air, under a pressure of
          25 to 30 millims., and which was enclosed by mercury in a tube 8 millims.
          wide, showed exactly the same lines as Plücker’s nitrogen-tube (<i>b</i>), except
          that some lines belonging to the spectrum of mercury also appeared.</p>

          <p>This observation may be regarded as a confirmation of the conjecture above
          expressed as to the condition of Plücker’s tube III. (nitrogen). In the observations
          described under <i>b</i>, the air saturated with aqueous vapour was under
          a pressure of 22 millims. Besides the sodium-lines, lines of the mercury-spectrum
          appeared at 6·25, 7·59, and 15·71. The spectrum of rarefied air
          under similar pressure was found to accord completely with the spectrum of
          the light in the broad part of Plücker’s tube.</p>

          <p>III. (Nitrogen <i>b.</i>)—A comparison of the spectrum of rarefied air saturated
          with aqueous vapour with the former shows the striking alterations in the
          spectrum which are brought about by the presence of the aqueous vapour.</p>

          <p></p>

          <h4 class="h4-appe">Comparison of the Aurora-Spectrum with the Spectra of Atmospheric
          Gases and of Inorganic Substances.</h4>

          <p>In the next place, I turn to the comparison of the observed spectra of
          different gases and of the air with the spectrum of the Aurora. The first
          band of light in the red part of the Aurora-spectrum most probably coincides
          with the first system of lines in the spectrum of nitrogen (<i>a</i>). Probably only
          the bright part of this group of lines is perceptible, on account of the
          extreme faintness of the Aurora; and as in nitrogen the increase of the
          brilliancy of the spectrum takes place towards the violet end, the absence of
          the intermediate spectrum towards this direction would find its explanation.
          The most intense line of the Aurora-spectrum at 7·12 is to be also found in
          the spectrum of nitrogen (<i>a</i>)—as a very faint line, however. That this line
          appears in the Aurora by itself, and with intensity relatively great, need not
          appear strange, considering the great alteration of the gas-spectra under
          different conditions of pressure and temperature. The third line of the
          Aurora-spectrum, very vaguely defined on account of its great faintness,
          coincides in the same way with a nitrogen-line.</p>

          <p>The line at 8·71 is met with in the nitrogen-spectrum (<i>c</i>), as well as in the
          air-spectrum (<i>b</i>). The third line of the oxygen-spectrum at 8·95, which
          <i>seems to appear under very different conditions</i>, is found again, as the fifth
          line in the spectrum of the Aurora. <i>Moreover, the sixth line in the Aurora
          at 10·06 coincides very exactly</i> with the known nitrogen-line appearing in the
          spectra of some of the nebulæ. Lastly, as to the broad band of light in the
          Aurora-spectrum from 12·33 to 12·88, several lines are found in this place in
          the spectrum of nitrogen as well as the air-spectrum (<i>a, b</i>); so that here, too,
          a coincidence between the spectra may be regarded as probable.</p>

          <p>The observations show with some certainty that at least one line at 10·06
          of the Aurora-spectrum coincides with the maximum brilliancy of the air-spectrum,
          and that the other lines appear with great probability in the
          spectra of atmospheric gases.</p>

          <p>In the very great difference of the gas-spectra under varying conditions of
          pressure and temperature, it would indeed be difficult to succeed in producing
          artificially a spectrum which should resemble that of the Aurora in all
          parts. Moreover, it must be admitted, under the hypothesis that the Auroræ
          are electric discharges in rarefied air-strata, that these strata, qualified for the
          transmitting of electricity, will have a very considerable thickness.</p>

          <p></p>

          <p>In this case the conditions of pressure on these air-strata are themselves so
          different that, within certain limits, each will yield its own peculiar spectrum;
          but we shall see the sum of collective spectra, so to speak, spread out behind
          each other; and therefore the impossibility of attaining a perfect agreement
          between the Aurora-spectrum and the artificially exhibited spectra of mixed
          gases is evident.</p>

          <p>A comparison of the Aurora-spectrum with the spectra of inorganic substances
          may be easily worked out by the help of the above-quoted wave-lengths of the
          single lines of the former, with due regard to probable errors, and with the
          aid of Ångström’s Atlas of the Solar Spectrum. Here the perfect harmony of
          the brightest Aurora-line (which was fixed with an exactitude of about one
          seventh of the separation of the sodium-lines) with the lines of the iron-spectrum
          is especially striking. The wave-lengths in the above-cited observations
          of the bright Aurora-line vary between 556·9 and 557·3, whilst,
          according to Ångström, two lines of the iron-spectrum are situated at 556·85
          and 557·17.</p>

          <p>Iron-lines corresponding to the other Aurora-lines, within certain limits of
          accuracy, are also to be found, as will be seen from the following comparison:—</p>

          <table summary="" class="borders">
            <thead>
              <tr>
                <th colspan="2">Aurora-lines.</th>
                <th>Lines of the<br />iron-spectrum.</th>
                <th>Remarks.</th>
              </tr>
            </thead>
            <tr>
              <td rowspan="2" class="bl nobr"></td>
              <td rowspan="2" class="tdr">629·7</td>
              <td class="tdr">630·08</td>
              <td colspan="2" rowspan="2">Moderately bright.</td>
            </tr>
            <tr>
              <td class="tdr">629·85</td>
            </tr>
            <tr class="spaced">
              <td rowspan="4" class="bl nobr"></td>
              <td rowspan="4" class="tdr">539·0</td>
              <td class="tdr">539·60</td>
              <td rowspan="4">Mostly very faint.</td>
            </tr>
            <tr>
              <td class="tdr">539·92</td>
            </tr>
            <tr>
              <td class="tdr">539·05</td>
            </tr>
            <tr>
              <td class="tdr">538·85</td>
            </tr>
            <tr class="spaced">
              <td rowspan="3" class="bl nobr"></td>
              <td rowspan="3" class="tdr">523·3</td>
              <td class="tdr">523·43</td>
              <td>Very faint.</td>
            </tr>
            <tr>
              <td class="tdr">523·21</td>
              <td>Moderately bright.</td>
            </tr>
            <tr>
              <td class="tdr">522·90</td>
              <td>Very faint.</td>
            </tr>
            <tr class="spaced">
              <td rowspan="5" class="bl nobr"></td>
              <td rowspan="5" class="tdr">518·9</td>
              <td class="tdr">519·79</td>
              <td><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="tdr">519·40</td>
              <td><span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="tdr">519·16</td>
              <td>Moderately bright.</td>
            </tr>
            <tr>
              <td class="tdr">519·06</td>
              <td><span class="ditto2">”</span> <span class="ditto2">”</span></td>
            </tr>
            <tr>
              <td class="tdr">518·51</td>
              <td>Very faint.</td>
            </tr>
            <tr class="spaced">
              <td rowspan="5" class="bl nobr"></td>
              <td rowspan="5" class="tdr">500·4</td>
              <td class="tdr">500·65</td>
              <td rowspan="5">Very faint.</td>
            </tr>
            <tr>
              <td class="tdr">500·52</td>
            </tr>
            <tr>
              <td class="tdr">500·49</td>
            </tr>
            <tr>
              <td class="tdr">500·30</td>
            </tr>
            <tr>
              <td class="tdr">500·20</td>
            </tr>
            <tr class="spaced">
              <td class="bl nobr">From</td>
              <td class="tdr">469·4</td>
              <td rowspan="2" colspan="2" class="bb">3 stronger and 4 very faint iron-lines.</td>
            </tr>
            <tr>
              <td class="bl nobr bb">to</td>
              <td class="tdr bb">462·9</td>
            </tr>
          </table>

          <p>Yet this agreement, though remarkable, can only be considered as complete
          proof of the presence of iron-vapour in the atmosphere when we shall have
          succeeded in showing by observation analogous modifications of the relative
          conditions of brilliancy in the iron-spectrum by alterations of temperature
          and density; and in this way explain the appearance of relatively very faint
          iron-lines in the Aurora-spectrum, or, on the other hand, the absence of the
          most intense lines.</p>

          <p>It will meanwhile remain far more in accordance with probability to
          regard the <i>Aurora-spectrum as a modification of the air-spectrum</i>; since we
          are already aware, in the case of gases, of the alteration of the spectra by
          conditions of temperature and pressure; and an agreement, at any rate, quite
          as certain between the spectrum in question and the spectra of atmospheric
          gases has been proved above.</p>

          <div class="smaller">

          <p>[I am indebted to Miss Annie Ludlam for a translation from the German
          of the above Memoir.—J. R. C.]</p>

          </div>

        </section>


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